Welcome, If you got hooked on flat heads with the 45, you'll just like the 74/80 more. As others here will testify, the BT flatties will definitely keep up with the pack. Parts interchange for all years, and some engine parts interchange with 45's. Oil pumps, tappets, come to mind. And a BT SV will fit in any big twin frame made, till modern times. That is to say, up through shovels. Drive train is the same as OHV. Just about any part you might end up missing is available aftermarket. There are lots of books out there, but you can't go wrong with the reprint of the Harley 41-47 big twin repair manual and the 41-54 parts manual. It's all we had in the old days and if you read them cover to cover a couple of times you'll have a good knowledge base to start from. Plus, of course, the webboard here has lots of people who have worked on these babies for years and know all the kinks.
And don't worry about the bike's value. If you build one of these babies, build it to suit you. Hell, let your heirs worry about the 20% beating they'll get on the selling value 'cause you didn't restore/build to exact stock specs!!