you could write to the DVLA and ask for the history. You'd then see how long the Black Cab Garage owner had it. If you are sure he did the conversion and he had it from pre '95 you'll probably be fine, no reason i can see for it not being legal.
The chassis age is kinda irrelevant anyway. Put a new chassis on yer series now, doesn't change the year on the V5 or the tax status.