Ok, looking for some constructive advice and practical information. No, quick retorts without some sort of qualifying statement.

My scenario:
I have a solid '73 350 block and a '65 425 block. I have a rebuilt set of cylinder heads for both of them.

So, do I take the 425 or 350 to the machine shop?

I will need to purchase pistons, have the connecting rods balanced and have crankshaft turned/balanced at a minimum as it relates to machine work. And that applies to either situation.

My main concern is: will all my engine accessories that I have currently from the SBO 350 I have installed work with the 425? Or will I have to make adjustments (other than motor mounts), which will incur more costs?

The only other thing I will have to buy that I know for sure is an intake for the 425 if I go that route, as I don't have one on hand.

I understand more displacement is attractive, but I want to make a smart decision from the get go. I am not afraid of building a solid/reinforced 350, so I am open to all input.

Oh, and I am looking for a reliable street motor that will impress when needed but still handle the 700 mile round trips that I have grown accustomed to with my current Olds 350 power plant (with the original 1970 bottom end of motor) which also handles 1970 aftermarket air conditioning to boot.

Any practical advice or thoughts are appreciated.

d1

 

Pretty sure all the '70 SB brackets and such will work on the 425.

You may want to use headers or W/Z exhaust manifolds like a 442.

350 engine kit will be spendy, 425 somewhat more so.

Not that much cost difference I think but have not bought lately.

You will want to be SURE you get the correct cam Lifter Bank Angle for that D? or A block 425. Probably the 45 degree block. Or, procure a 455 block and use your 425 crank and rods and new 425 pistons and such kit.

Get an aluminum intake either way, right?

You have a flexplate for the 425? they do not interchange to a 350.

Most other parts interchange
oil pan
timing cover
water pump [must match your pulley set]
rocker covers
distributor
oil filter adapter [exc. Toro which was not made in '65]

Your motor mounts will not differ, they do not care if you have BB or SB.

You will enjoy a fresh new 350, for sure, but your Grin may be somewhat larger with the 425. Until it's time to feed it.

What number 350 heads, what letter BB heads do you have?

 

Thanks for the reply. I totally forgot I needed exhaust manifolds for the 425. Unfortunately I sold mine to fund another purchase.

I have #6 heads for the 350 and A heads for the 425.

I am pretty sure the 425 will be more expensive to build. But, if the result is worth the additional funds I would be ok with that.

d1

 

The result of a mild 425 will be better, the cost is not much different at the machine shop. Some of the parts maybe more expensive. You also have to consider possibly added cooling, and stiffer front springs. All in all there is nothing like popping the hood and admiring a BB. Headers are a must.

 

I already have heavy duty springs up front. And I have a 4 core aluminum radiator, so that should suffice, but I have seen plenty of electric cooling fans added to BBs. I am concerned about the headers fitment. My 350 Sanderson shorties were a pain in the *** to install as it was, long tubes I assume would have their own challenges.

But I understand your point of view. Thank you.

 

Header fitment on 68 and up is a lot better than 67 and back, much more room. There is also more room for a BBO over an SBO.

 

Just remember you can't put headers on a Supreme

ha ha ha ha - long running Oldsmobile joke

We need a forum for Olds Mythbusting
Headers/ Supreme, SMW403, "points are not adjustable" etc.

But, seriously,
Kanter has pretty complete kits
HOWEVER your A heads use different rockers. Swap for C heads or later, or maybe jump to roller rockers that use your A heads' 3/8" threaded holes.

https://www.kanter.com/productdetail...Router=Gallery


$1120 for a 425 Basic Kit PN 088568B

1968-ish 350
088580H at $1200 or so vs

$1135 for the lower compression kit.

Really, negligible differences there
Machine shop costs will be about the same.

The 425 is the clear winner all the way around, as long as you have a flexplate and balancer for it. You will not be disappointed. Torque on Tap.

Some extra costs with the rockers, perhaps, and you need the $250 manifolds or a cheap flawed pair if you can live with that.

 

Yeah, I will probably build the 425. It may take me a little longer though, but I should have it done by next spring. I will use the A heads and will have to find the proper intake manifold and camshaft for what fits my needs. I will be researching headers in the mean time.

Thanks for the feedback.
d1

 

Keep this quiet, but I think you'd be better off with the 425. I am a small block guy at heart so that is a hard statement for me to utter.

What is your budget, though? I do not see that mentioned.

 

I recently had to find a flex-plate for my '67 330
which should be the same as the one you need for your 425.
I posted on this site and got several replies,
so finding the flex-plate you need shouldn't be a problem.
'64 330 and all '65-'67 Olds V-8 should be the same.

The 425 will be a lot more fun and
cost of the build won't be much different from the 350.
To me, the extra fuel consumption is worth it,
depending on how you intend to use it.