OK, not going to get into the stuff about-"where the alternator, or a/c mounts" I am not up on that stuff, but as far as the fuel system, terms get throw out and misconstrued, so here is a bit of info.
First, early Astros(and others) were carbed, or TBI, throttle body injection.
Then came CPI, central port injection.One central injector, with 6 poppet valves.
Then,96 and up, came SPFI, but more precisely--Central Sequential Fuel Injection (CSFI) system. With a single injector for each cylinder, the CSFI system has a much more efficient design than its predecessor.
A great system, but more difficult to service.
Interestingly,a CPI, electrical plug is exactly the same as a TBI injector plug, also a cause for problems when older, have repaired/replaced quite a few.
So, as far as "fast and easy", not swapping wiring harnesses, dropping in a 350 throttle body, add jumper wires to the injector, as both fire at once anyway, and swap in a 350 ECM.
Now that will give you more to think about.
Going to a CSFI will require a harness swap, but, generally, make more HP, run cleaner, and, engines last longer, as they do with any/most fuel injection systems.