Welcome new guy. Great forum here. Your questions about interchanges should be answered by the people on this forum. Although I have not done either of your swaps on my 66, I've read that the disc brake swap is not difficult and there are many possible donors for parts such as newer cars like 72-79 Torinos, T-Birds, LTD II,s etc. The 429 will make a fine powerplant for your Gal also. I want mine to stay original, so the 390 stays, but there is nothing I would like better than a 429/460 swap.