April 83 should now qualify based on registration so you do not need to worry about the build date. It is a very easy process you just take you v5 and a completed v112 (mot exemption) to the post office and they fill in and send off to the dvla. a few weeks later you get a new v5 with the tax class as historic. I have done both my series and tractor in the last couple of years. it is far easier than people make out. i am not even sure you need the v112 as they don't take it from you or send it away but it is easy enough to complete and take just in case.