MaSnaka John, Overdrive trannies are nice IF you have the right motor and setup. Need a motor with alot of lowend torque at 1,500 rpms to 1,800 rpms. What endratio do you have? Is your current trans a wide ratio or narrow ration trans?
Most FE motors make great torque down low. It depends on the camshaft. It may be simpler to change the rearend ratio that swap the trans.
If you lug the motor at low rpms, over time you will damage the drivetrain. The clutch will only absorb so much vibration of the motor lugging.
Another main thing and this to me is the biggest,
OIL PRESSURE. What is the pressure running at theses low rpms. FE motors have poor oiling systems. If you are running 50-60 psi at 1,200 rpms, this is fine. If you are only running around 30 psi, IMPO, you are starving the back of the motor of lube. Yes I know that the motor only needs 10 psi to float the crank shaft and in some motors like an LS1-LS( that's fine ). This is not a gm motor.
If you get a trans with the right ratios that's great. getting a final 5th gear of .65 and a low rearend ratio in the low 3.00's you are not going to be happy.If you do go though with this get a final ratio of . 87 for 5th gear.
You you going to be cruising all the time?
Last note, TKO have come along way to making a better trans. Need a fair amount of parts to swap into your car. May even need to replace the drive shaft.
If you have time, look into a Richmond super street 5 spd with OD. This trans will also hold 600/600 of HP and torque. Direct swap. May need to change just the clutch disc and pilot bushing in the back of the crankshaft. Top loader and Richmond are the same length. Shifter location same hole. TKO you may have too change holes to get it to fit. Different shifter between old and new trans. Some thing to check out the think about. Good luck Rick L.