TR4A should fit, but is not as strong as TR6. Probably work fine if you drive modestly, but I wouldn't try it if you like to drive hard (as I do).
The 4A used the same clutch as TR6, so no worries there. If you did get a TR4 (instead of 4A), the only clutch change necessary is to trim a bit off the input cover (and use the TR6 clutch components, TOB, carrier sleeve, etc).
BTW, I think your "protocol" is fine. Maybe could be just a bit more descriptive on the title line, like "TR4A trans in a TR6?" but what you have is good too. It can also help if you put your commission number in your signature; for example late TR6 uses an adapter at the rear motor mount for a non-overdrive gearbox; which might be missing if you don't have the old gearbox.
PS, Welcome to the asylum!