Firewalker

Many people today don't understand modern overdrives, or that you don't put it under heavy loads such as towing without the risk of burning them out. Things can be made stronger but they still have there limits, and the stronger or harder you make one part, then that can add stress elsewhere.

TexasT

Now you drew me offsides. The 8.5 ten bolt the cutlass(and skylark) came with in 71-72 is an excellent rear with parts availability and strength rivaling the 12 bolt chebby rear. Upside is the cost is less if you already have one, and it has bolt in axles unlike the 12bolt so you don't have to add a c clip eliminator. Twelve bolt(and 9 inch) is heavier and takes more power to spin it. I'd go to a nine inch before i'd spend the coin on a 12 bolt if I wanted to upgrade but for the power i'm doing I'll stay with the 8.5 ten bolt. Plenty of nine sec cars with the ten bolt.

As far as the oiling systems, tech has moved forward. Those 455 engines were engineered to get aunts and grandmas to the market and church in their 98. The good fortune that GM let the divisions put together super cars didn't afford those divisions the budget to re-engineer the whole engine program. They made due with the available parts they could get. A few came back for warranty out of thousands or tens of thousands that hit the street doesn't mean the stuff was junk. It means there were plenty of meat heads whose dad fronted em a car and they tore it up. Same happens today.

Firewalker

Th real difference in strength was the size of the axles which looked like toothpicks compared to the Ford 31 axles, and the C clips on the 12 bolts didn't help. The really strong Ford rears seen in nitro cars were the 9 3/8, which many mistook for a Ford 9 inch.

But the other reason the Ford 9 inchers were popular was also for fast gear changes. Then the aftermarket stepped in to strengthen the weaker 9 inchers, which did have a tendency to break past a certain power level., and they never did a thing for the Ford 9 3/8s nor brought out gears for them. Ford almost always put a 2.80 gear in them. The Dana 60s were consider by many to be the strongest and heaviest, but I have seen them break as well.
Glad to see you TexasT.

Brians1

So again thank you for the feedback and somewhat of a history lesson LOL

My 72 currently is a stock 350/TH350 with the (I assume ) 8.5 Ten Bolt...Still has the original Frigidaire AC compressor that works too
My new 455 (500+ HP/550+ TQ estimated on paper) and TH400 will be placed in...new drive shaft will be purchased....17 in tires...

So sounds like I can just swap out the guts of my 8.5 ten bolt to a better gear ratio such as 3.23 or a 3.08, bolt the cover back on and light up the tires...yea ?

** I haver already spent a ton of money that the wife so far hasn't asked about...so im trying to fly under the radar for now LOL

Brian

olds 307 and 403

Good to know about the 4L80E overdrive clutches being robust, even though there are only two. I actually towed my boat with a 2004R for quite a few miles in OD, never toasted it either. Also somehow my 42RE has lived but I lock out overdrive and only tow a mile to our local reservoir and never reach 50 mph. I disagree on the Type "O" differential being a POS. The 70 has the better oiled axles and 71-72 10 bolt with bolt in axles areare hig sought after, having even better axles and Chevy 8.5" internals. Brian Trick prefers them over the 12 bolt, supposedly the stock 12 bolt axles are spindily junk. I think the 455/TH400 and even the stock ratio to start is a decent starting point, usually a 2.56 or 2.73 stock. Drive with that first and see if you want it revving higher on the highway. It really depends on usage, everything major center is 1.5 to 6 hours away up here. Our Challenger GT with the 8 spd auto spoils us with 33 mpg and 1750 rpm at 70 mph with every option available. It does make a drive very nice.

Oldcoyote

How tall are those tires?

Firewalker

Your rear should be fine and it will take a lot of power and abuse, but you are not planning anything close to its limits. The problems really show up, with a lot of power, with good suspensions for weight transfer, and really sticky tires. This is true of most stock rear ends, and even more so today, with most people using automatics rather than 4 speeds.

Brians1

olds 307 and 403

P235/55R19=29.1" tall. My 70 Cutlass with 2.78 and the same overdrive ratio with 26" tall tires will be at 1900 rpm at 70 mph.