David DeCan

Considering the conversion of my 70 Cutlass Convertible TH350 with front bench from a TH350 to Muncie M20, buckets and console. Have purchased 4 speed console and console mounting brackets. Will purchase 4 speed shifter hump for welding onto the transmission hump.

I think I understand the best way to cut the hole for the shifter to come through the floor board---start small and continue to cut where the shifter hits the floor board from the underneath side until the shifter is completely through the floor board and fully operative. This process should be accomplished with the manual trans bolted to the bell housing and the bell housing bolted to the engine. All of the above should give correct floor board cut placement and proper fitment. If my process is flawed here please correct.

So here is my question: How do I properly place the new manual shifter hump for welding? The shifter is poking up through the floor with minimal opening to allow its correct operation, but now I need to weld the shifter hump to the transmission hump--how do I get this lined up property?

What I don't want to happen is it gets welded and is off by a smidge affecting the install of the console, thus affecting the install of the buckets.

Tied in with this shifter hump placement question is the placement of the console brackets to be welded to the floor board? Placement of brackets properly concerns me. I can see this whole process going very badly. Like a bad building foundation affecting the build from the first board to the tippy top of the roof.

I am having a mechanic/welder do this conversion for me and I do not want him to screw it up. He's a good mechanic but I need to demonstrate to him how I want him to place these parts for welding.

If someone has done this and it worked properly--perhaps you could address the welding of the bucket seat brackets that will need to be installed to convert from a bench to buckets as well.

As always thank you for your assistance.

David

My442

What I did was put the transmission in with the shifter stub, and mark where it hits the floor from the bottom.

Then, drill a small pilot hole, and remove transmission.

Then you can cut the hole and mount the hump.

The pictures are of factory parts form a 1970 car.

David DeCan

The hole in the transmission tunnel is massive! They must of had a process for placing the shifter hump???

Are those holes in the shifter hump placing the console?

My442

That is a factory hole.


The cars coming down the line had the holes cut out with a torch.


The hump was welded to the floor over the hole.


The hump and donor floor were welded together when I got them.


I removed the welds to separate them.


Do you need a hole that big?


No.

My442

As one of the few folks here who actually worked in a GM plant back in the day, I will tell you that the line worker used a fixture to locate the hole and the hump.


Human discretion was taken out of all of the assembly steps.

David DeCan

That's what I was thinking. So how do I do this without a fixture?

My442

See my post above.

Use the shifter stub to see where it hits the tunnel.

It needs to be in neutral.

Drill a small hole, and from the interior use a Sawzall or a hole saw to cut.

The hole you need has to be approximately the size in the photo.

The reason is that the shifter linkage is biased to the driver side.

If you want to discuss, PM your number and I will call you.

My442

Use the hump as your template.

It is easy to do.

David DeCan

Is the shifter hump a part that can be removed from a four speed car and installed in another car--or is it pretty much destroyed upon removal? The Parts Place has a new one. Is the Parts Place shifter hump a good piece?

Not really following how the hump gets lined up for welding on the transmission tunnel, but perhaps my confusion is because I don't have parts in hand. Also having a hard time wrapping my head around the size of that trans tunnel cut out and placement of the shifter hump. When I get a little closer I will PM you "My442" so you can walk me through it. Thanks for your help.

My442

When I got my donor floor and hump, they were spot welded together.

I separated them without destroying the hump.

Another option is to use short sheet metal screws to attach the hump.

Give me a shout, and I will walk you thru it.

Brian Trick is also very knowledgeable on these swaps as well.

David DeCan

Thanks to all who have patiently been trying to walk me through this.

I have been thinking about this for two days now.

Without a factory assembly fixture, placement of the shifter hump on the transmission tunnel seems to be trial and error-----UNLESS the shifter itself aligns and places the shifter hump. Yes/No??

If this is the case then install the transmission, use the shifter contact point on the trans tunnel to cut through into the passenger compartment. Then fully install the shifter, boot etc and attach the shifter hump to the shifter, boot etc-----Then spot weld the shifter hump to the transmission tunnel.

Thus the shifter itself is aligning and placing the shifter hump on the transmission tunnel.

Sounds logical to me, but is it right?

My442

It's logical and not hard.

You are using a console, yes?

David DeCan

Yes using a console. Thx

My442

If the car has a console now (automatic), the manual transmission modification can be done to it.

The placement of the hump can vary a bit as the console mounting does not use the hump for attachment.

The factory assembly manual has very detailed drawings of every aspect of this.

Are you going to use:

1. Clutch safety switch?
2. Back up lights?
3. Reverse lock out?

Be aware that the 70-72 4 speed cars used a unique shifter handle, P/N 7169.

The factory Hurst shifter base, P/N 3138 did not have a stop bolt, and used rubber grommets instead of hard bushings.

David DeCan

Clutch safety switch--no
Reverse lock out--yes
Back up lights--yes

70 Convertible car is an auto on the column. NO console--yet. Will be adding console, buckets and 4 speed at the same time.