Here's a new Clutch Master Cylinder (5/8") for the following models from Triumph -

TRIUMPH

Triumph 1500 FWD     (1970- 73)

Triumph 1500 RWD     (1973- 76)

This has the correct 7/16" thread for the pipe and the correct 5/8" cylinder bore.

You can buy a cheaper Clutch Master Cylinder off ebay for £22, then spend the rest of the day looking for an adaptor

OR

You can buy the correct one here, fit it and drive off - You decide.

You may need to used your old pushrod - It's easy to swap as it's only held in place by a circlip.