If you by any chance, changed or did not do a trans TV cable check that will affect trans operation.
The whole condition sounds like you have multiple overlapping issues. Are you checking your work and following provided install instructions. Use GM service manual for standard procedures. Twice in forty plus years I saw headers, exhaust manifold cherry red on start/ idle the ignition/ firing order screwed up. That was scary, holy *****. That was then, neighbor's kid. Just one problem it could be. Good luck.