Firstly, welcome to the Forum.
Its the 350cc version which seems to have been called t21 and then changed to 3ta in 1959?
The unit construction (gearbox in a compartment attached to the crankcase, rather than being separate) 350 twin was new for 1957, its model name
was just "Twenty One" because 1957 was 21 years of the Triumph Engineering Company - founded in 1936 to rescue the motorcycle-making of the earlier Triumph company - and 350 cc is 21 cubic inches in the US. The model's code
That first 350 spawned 5TA "Speed Twin" (500 cc), T100A "Tiger 100" (sporty 500), US-only TR5AR and TR5AC "Trophy" (sporty 500), and T90 "Tiger 90" (sporty 350). The names were all from superseded pre-unit (separate gearbox and engine) models/versions/variants, the "A" was added to the model codes because Triumph at least supplied parts for pre-unit 3T, 5T, T100 and TR5. "T90" had previously been the model code of the pre-war 500 cc "Tiger 90" single but all tooling and drawings for the singles had been destroyed with Triumph's then works in Coventry by the infamous bombing Blitz of 14th/15th November 1940.
Reason im asking is I want to find a bobber bolt on rear end and can find a 500 bolt on
The whole unit 350 and 500 range was known by Triumph as the "C-range" (singles being "A", pre-unit twins and unit 650's being "B"). By-and-large, major parts - frame, swinging arm, crankcases, etc. - are common to all models and variants in the range. So, yes, a bolt-on rear-end from a unit
500 will fit your 21.
did a 5ta which from what ive read was the same bike as 3ta except for the top end?
Yes. As traffic generally got faster through the 1960's, the basic 18 bhp 3TA became a bit of a slug; common upgrades were either a 500 top end or Tiger 90 bits. Not so easy to convert a 3Ta these days.
I'm new to Triumphs and would like to convert a 1958 T21 I just bought.
Reason im asking is I want to find a bobber bolt on rear end
Depends how complete and original the 21 is; the more complete and original it is, the more I would not - complete and original bikes are rare, already-mutilated Triumph twins are still pretty common. Keep an original and complete bike original and complete, or pass it on to someone who wants an original and complete 3TA; "bob" an already-mutilated twin, and you'll likely prefer a 500 for riding in modern traffic.