Clutch Issues - No Burnouts

Something is binding when the engine torques in it's mount.

Have you checked your motor mounts to see if they haven't failed and the engine is moving too much?

Also lube all the pivots and moving parts.

There are kits for the nylon bushings in the z-bar, could be they are worn.

Are all the parts specific to the car or are you using parts with an unknown pedigree.

I had the same problem and in my case it was the fact I was using a 72+ bellhousing and a 72 and down z-bar pivot bracket on the bellhousing. This had the z-bar sitting crooked causing the bind under power.