Okey-dokey, thanks to all who responded, much appreciated!
The 5mm is a tight/snug fit with drag, effort required etc. New belts are much tighter than the ones that came off were. I'm no expert (obviously) - but they seem in the ballpark; a lot happier with them as they are.
Sounds like that's probably fine. Which is what I thought, so thanks for the sanity check. Guess I'm gonna find out
I did take Brad's advice to turn the motor over a dozen times or so (probably more like 2-3 dozen now) on the new belts with some tension just to let them settle into where they want to run before tensioning @ TDC for each.
I think the thing that threw me the most was that I'm used to a "no-go" test being "nope, NFW does that fit" rather than "if you try harder, it'll still go", LOL.