I'd go for the 5TA - my old man had one (1964 engine and frame numbers, but registered as a 'D' in 1966, but with a slightly earlier (?) front end and tank (probably crash damage), and some nice aftermarket stuff like a John Tickle TLS front brake and alloy rims.
I think the pistons, carb etc. were to T100 spec and it went really well. It had whatever the prevailing version of the Boyer
ignition was about at the time (mid-80s), which never sulked or did anything daft because of battery