I know of a few people that are very good at that.
That's what happened to Eddie's car....
He had a good rebuilt trans in it, then bought a year correct trans and had John Cope do it and the kick down is wrong and it won't even shift right now....
He kept fixing it until it's broke....
Here's a list of what was wrong that he worked on:
Intake manifold not torqued.
Stud for throttle on carb was loose and wiggled...
Nut on stud for trans mount loose - you can see 1/4" of daylight above nut...
Coil + nut and wires loose....
Didn't install the trans dust cover that goes between the engine and trans so you can see the torque converter now... (We had one on it when we put it together two years ago...)
Header gasket leaked at flanges...
Kick down linkage not adjusted properly...
He used two thin gaskets with a metal plate for the carb base instead of the heat insulator 5/16" dual plane gasket that we originally had used...
The kick down in the trans doesn't feel springy like they do normally, it is free moving and then hits a hard stop and keeps the carb from opening the secondaries....
We spent all weekend fixing most of it and a few other things that were wrong, I really don't want to blame him, but it may be his fault also....