The thing about pre-1970s is fairly important. I bought my first plane, a #5, around 1982. I fettled it and it has a number of good irons for various jobs (and yes, you can change the effective angle pretty easily--with a microbevel), but there's one glaring and really annoying issue: the sides aren't square to the base. This makes it useless on a shooting board.
That's a simple check with a known-good engineer's square at the time of purchase. If you're making boxes, I'd expect you to be shooting mitres, etc. and not being able to use it on a shooting board would drive you nuts.
I have a #4 that's similarly challenged, too, and that one is Record, war-finish (so it's not just the later ones), but that one was never intended for shooting-board duties.
A decent #5 or 5 1/2 will be a go-to plane for years.
E.