[D.PASSMORE]

I'll throw this out here. Try this method, break the troubleshooting down to 3 areas: Motor, Transmission and Gearing. Ensure each one is functioning correctly and that it meets need for your application. ie: modified 4x4.
On the same note about your crate motor, your situation was similar to mine. I purchased a GM crate motor 295 HP for my 72 K20. I had a strong mildly built 350 in it that blew a head gasket, plus it had a lot of miles on it. My son needed a vehicle to drive to school (16yrs old at the time). I bought that motor, as it would be dependable, low HP driver for him. Plus I would rebuild my other motor.
So, as you experienced, what a SLUG! When I got the motor, I checked it out, two really lose rocker arms, fixed that. I went basic, EPS edelbrock carb and intake and NAPA wires and rebuilt point distributor. It ran great, I had to play with the jetting of the carb to get to run at max power. Even with the motor tuned (new dual exhaust), stout TB 350 and 4:10 gearing it was like driving my grandmothers 1976 Ford Granada with a four banger! Well, unfortunately he wrecked the old girl and we parked it. It has been setting for the last few years, only about 5k on the motor. Over the holidays, we pulled that motor and checked it out - good shape. Now, were going to use the bottom end and pump it up on the top end. I purchased some AFR heads, have a nice eldelbrock performer intake, recondition/rebuilt Quadra-jet and Taylor wires and a DUI small GM distributor. I still have to spec the CAM. So look for a motor build thread over in the drive train section. Good Luck!