If you wanna drop a 300 in it to be doing it, go for it.
If your looking to make some serious HP and TQ, keep the 390.
The 300 is what it is but if you warm up the 390, it can generate more TQ from idle to what ever rpm as compared to a warmed up 300 and actually give you respectable hp to boot.
Cheapest and easiest method to get the 300 rollin and generating respectable hp/tq, say 250-300hp and 350-400TQ is to keep it carb. If you go efi, then you'd wanna go with a stand alone efi controller, something you can tune the daylights out of or find a dyno operator and chip company thats willing to work with you. Another option is to use a mustang A9L computer but again, requires someone to reprogram the computer for your needs but relatively easier, as there is support for this eec.
Good luck
