I just purchased a 1972 240 with a rebuilt early 1958-1962 stock 283 with a five speed transmisson and 180 rear end. Claims to have 250 h.p. with 300 lbs of torque what can I expect for a zero to sixty and a quarter mile times with that set up. And how much of a differance would it make by installing the r200 with 3.90 gears, The car seems to shift threw the gears preaty fast now I shift from 2nd to 3rd at 50 mph at 4800 rpm 3rd to 4th at 70mph at 4800 rpm and into 5th about 95mph where would that put me with the 3.90 and would it hook up. Running 14 60 225 tires.

 

Can you keep the tires hooked up in first now? If not then going to the 3.90 gearing will just make it worse. I'd suggest looking for a 3.70 LSD R200 out of a 87-89 turbo Z31. You might also spend a few bucks and put the car on a dyno and find out what the torque & HP really are.
My LS1 equipped 280 has 311 RWHP & 326 RWTQ. That equates to about 365 HP at the flywheel. I have the LSD R200 and 225/50-16 tires and can't keep from burning them up in first and second.