By the way I appreciate any advice I didn't want to go all rogue, but I don't want to be the type of guy that goes out and buys a muscle car that's fully restored or the person who paid someone to build it, pull out a lawn chair sit in front of it at a show and represent that I actually know something about how it was built. That's not a hot rodder that's a guy with an identity crisis.
I'm going to tell stories of how and where I found parts, how I took advice on what gears to put in it :), how the first gear set was a mistake and I installed it wrong, learned from it , fixed it, then helped other people with the same, and then show people where my blood was spilt on it, where my knuckles were busted. That's who I am! I am only looking for responses from the same type of people, not armchair mechanics spending the weekend waiting for their car to get out of the shop!
So anyway what gear ratio should I use?
Classic!
The internals of 3.90 geared Th400's were stronger than their PX coded counterparts to handle the torque and horsepower.
I have a 3.55 in the 67...tachs out at 3500 on the freeway..not too bad.
IMO, a quality gearset from Richmond is only $250-300 including races, bearings and shims. Why not try the 3.90 and see how you like it with the M-21, both which, if I recall correctly, were ordered with your 68.
If you don't like the 3.90's..go with something numerically lower.
In my opinion, I would decide what transmission you are going with. From reading, it appears you are on the fence and people are trying to provide suggestions. What would make this easier is to decide which transmission you will go with first. Will the Th400 you have now hold up with 3.90 gears? Probably not for long. Will an M-21 hold up with 3.90? That is what it was designed for if the internals are good.
Either way...Th400 for now with XXX gears you may or may not be happy with..same with an M-21 with your existing gears. As you said, you have the drive to change gearsets out and can't be any harder than rebuilding an engine. Same holds true for swapping out a Th400 to an M-21 as long as you have all the parts. You can swap out the Th400 in 6-8 hours for an M-21. The only way you'd be happy is to actually install it and drive it...Or find someone with XXX gears with X trans...with "X" HP motor.
Either way you go..
