66 Dart 360 Swap Questions

Switching to a 360 in an early A from a slanty, is a whole new ball-game.
BTW
IMO, the hands down best solution to the exhaust system, is the IIRC5204 Hookers which split the primaries on the Driver's side to two down the inside and two down the outside, of the frame rail.
Second best would be the 340 HO manifolds of yesteryear............... but they are not any cheaper than the headers.

To me, it sounds like, you should probably sit down and re-evaluate your budget.
Successfully converting a slanty A-body to V8 power is not cheap in the first place, and a 360 into an Early A is even less not cheap. And! And living in California is super not cheap, either.
Better it is to find this out now, than after you have spent your life-savings on the project, and suddenly find out it wasn't even half enough.
AFAIK, 360s never came with 904s, so if yours comes with one, it is not off a 360 nor a 340. That just leaves a 318/273. And if that is what you have and it has not been upgraded to put up with the biggest smallblock Mopar ever designed, then it just a matter of time before it fails.
Similarly, the 7.25 while not made of glass, it is still a ticking time bomb. As is the wiener drive shaft, the cooling system, the fuel system, the ignition system, the brakes/suspension/ and steering, and on and on.
If yur already balking at the exhaust system for lack of financing, I see where this is going.
stuffing the engine under the hood is the cheapest part.
I hate to be the one to say this out loud, but somebody's gotta manup and be the bad guy, and it might as well be me.. When I did that same idea but into a later A, I'm into it for well over $10,000 thousand Canadian, just on the powertrain, which doesn't include extras like the Edelbrock heads or the GVod, nor any body/interior work, nor wheels and tires, nor any labor, which I do myself. From there it only gets exponentially more expensive.
What happens if the engine dies three weeks after you put plates on it? Or the Trans. Or worse if you crash it because the skinny tires and 9"drums couldn't get you stopped in time, or that stinking right front spindle snaps off, as two of mine already
did when I was in my twenties back in the early seventies, I was lucky, the damage was minimal.