My 781 heads were on a 454 from a 76 suburban. TH350 held up well also. The 7.5 axle has to go. You can take it out now or wait and break it on the first night out. I would still run the torque arm though. Build mounts off a 9'' or 12 bolt or Dana rear. Get the front mount off the trans.(You will only break trans mounts like this) and attach it to the trans x member. Dont screw up the pinion angle. BTW, for those with a more streetable car you can use the tailshaft from a vega to attach the torque arm to a TH350. They were torque arm also.
Not a lot of room for tires in stock wheel wells. 275/60 DOT or 26 x10 slicks.
3.89 gear is decent.