Gagster's 71 Swinger

Spent the weekend working on getting her ready for paint. Got the dash out, doors off, hood, fenders, etc.

Power washed the whole car, inside and out, cleaned out the cowl, cleaned out the trunk extensions, blasted down inside the quarters where the back windows roll down... everything is now nice and clean and ready to get started changing color. Before I can get crazy with a spray gun I still have to do a little work with some putty and a sanding block and I still need to pull the rest of the bits out of the engine bay. She still has brake lines, fuel line, steering column, etc in the way. This weekend I should be able to pick up some primer, finish removing bits that don't get painted (or get painted separately like hood hinges) and maybe make some visible progress. I'm really excited.

Question though:

When painting an engine compartment, what do you do about the old paint? Is it necessary to sand blast all the old paint off? Seems kind of like overkill. Is there a certain kind of primer that's better at covering old paint as opposed to bare metal? Is there some prep to do to the old paint besides making sure that it's clean? I suppose I could get in there with a small sanding block and rough it up but the engine bay is such a complicated shape that I'm sure there'd be a bunch of places I couldn't sand properly. Same question for the inside of the trunk really. It seems like there's no good way to strip it so how do you paint over it properly?