360 driveability problem

Well it could be a few things.

The cam timing could be off a bit. Retarding the timing would make it soft down low.

It could have more cam then you think it does and be under geared and not have enough converter. This could be lugging the motor down low and causing the detonation.

It could be a timing issue. Try giving it some more timing at idle. My distributor is set for 18* initial.