When starting vehicle and putting in reverse or drive, shakes badly and shuts down.

I had a valve seat drop and completely rebuilt the engine with an aftermarket head, reinstalled engine and transmission and all other parts, when starting I get the (!) symbol on the dash, installed a new battery and still comes on, light comes on at startup right away when not even hot, when idling and A/C on engine idles rough and almost shuts down probably a vacuum issue. But my real concern is when it is idling and I put it in either reverse or drive with foot on brake it shakes like crazy and shuts down so please tell me what is my next step of diagnosing these issues. thanks