I can speak to 1 and somewhat lesser to 2.
1) Unless your heads are pretty nice already you're gonna have close to E-Head money in them, particularly if increasing valve size. I would not do it again. Granted, you're going to want to check the E-Heads over, but still...
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2) Likewise it depends on what it needs, pistons, rings, bearings, arp bolts, re-size rods, grind crank, balance... bet you are at least halfway to a stroker kit if not three quarters... My situation was a little different as I started with a 360, but I've always said if I had it to do over I'd build at least a 410... with the current state of strokers, if you're gonna build a 410, you might as well build a 445...
3) You didn’t ask, but a two owner, one family Fairlane GT sounds like a car I would probably not make any irrevocable changes to...
You’ll probably get a lot more opinions and more expert too...