I'd place the ETT as a higher priority than the seat tube (angle), as long as the seat tube angle is between 73 and 74 degrees.
Even higher priority would be the stack and reach of the bike (if listed), but I suppose you'd have to have quite a history of riding bikes, and have those figures down pat.

One degree diff in the seat tube angle should still allow you to place the saddle fore or aft to get the right fit.
Only thing is, if the saddle rails end up being forward in your fit, then you won't have room to connect one of those small bags at the back of the seat rails.

I don't suppose you could try tacking down a geo chart of that old Harry Hall frame? (and compare it to the frame/s you're looking at)