Tim, if the distributor is one of 123 Distributors (the one on my 1969 MGB is) then the RVI-type tachometers manufactured by Smiths Company apparently won't work properly. But they can be adapted - if is a 123 check out their website for details. When I fitted a 123 (well watched whilst someone else did
) he just replaced the old dizzy with no other amendments and the tacho has been fine ever since.
From memory there is also some info re electronic distributors and tachos on the MGOC Members Forum - don't know if you have access to that? If not, let me know and I'll see if I can find the articles on it.