91 RS 5.7 swap 6 speed currently has 308 gears those gotta go . Was wondering if 342 gears would compliment tpi better or would 370 work better with tpi and 6 speed . Anyone with a similar swap? I know I'd be better off ditching the tpi but I gotta take some short cuts for time and money reasons , thanks.
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I have a 4.8 with a LS T56 and 3.45 rear gears, I think they are the optimal ratio for an everyday use car, as did GM. In the future I do have a set of 3.7 gears to put in it but I may just stay with the 3.45 since I am now planning for a lot more power with a 6.0 I think the 3.45 will still work well.

 

Either would work fine, but I think 3.70’s are a little better match, but definitely no more gear than that. I am doing 3.70’s with a T56 and a ZZ6 350TPI. We’ll see how it is when it’s all together, but it should be right at or slightly past the power peak rpm at the end of the 1/4 mile.

It should be ok on a stock 350TPI as well. 95mph trap speed would be at 4600rpm.

The T56 has a tall 2.66 1st gear, so the 3.70’s should match pretty well. Also, the 0.50 6th gear is better with 3.70’s than 3.42’s.

 

Right on thanks for the imput got a 9 bolt but it's a 277 from a parts car was thinking about swapping it to 3 series 370 or 345 unless I can find a good used 3 series disc brake rear end somewhere near Ga.

 

Building a 9-bolt is a lot more expensive than a 10-bolt, especially since you will need to source a 3-series carrier. I built my 9-bolt with 3.70 gears and rebuilt the existing 3-series carrier. It's done and it will be fine, but I kind of wish I just bought a whole new rear end assembly.

You could just rebuild/upgrade your existing 10-bolt with new posi carrier, gears and bearings for 1/2 the cost to build the 9-bolt. Leave the drum brakes. The 3rd gen rear disc brakes aren't that great anyway.

If you did find a good 3.45 9-bolt or 3.42 10-bolt that was in good shape, I would just run that as is.

Alternatively, there are a few complete brand new rear end assemblies available with whatever gear ratio you want, but they are pretty expensive. After what I spent on the 9-bolt and the time I invested in the rebuild, I kind of wish I just went down this road.

 

4.10 was the common upgrade on the 4th gens with the T-56 although they may overshoot the TPIs power band a little as the LT1 and LS1 both like to spin. 3.90 might not be a bad gear. 3.42 is a bit tall for 6th unless you are cruising 85+. I find it easier to launch with a lower gear less rpm and less abuse on the clutch, less wheel speed if the tires slip which causes you to gain traction more quickly and improves the 60'.