Transmission and bolt patterns on '66 I6

I am looking at a 66 mustang, it has a I6 with a automatic tranny. the seller does not know the type of tranny or size of motor, I am thinking a 200cid and a C4 transmission.

If this is true, what I am wondering, is if the bolt pattern on the transmission bellhousing would mate up to a 302 without problems, and if there would be any problems dropping a 302 in that car and what might be needed.

I know that it would probably need an small upgrade to the front suspension to account for the extra weight, but would any major modifications need to be done?

Also a quick quistion, but does 6,000 sound ok for a 66 stang, no rust on body, minor underbody rust, interior is really good if not excellent. I6 w/ auto and AC, daily driver

Are any of those parts interchangeable through over a few year models? if so which years? reason I ask is that I have access to a few parts cars, just not a 66, I need to look into all possiblities before I agree on anything.