maybe if i refer you to a conversion by a member here, it will help and give you some ideas about your project. okiflyer (the member) has installed a VW 1.8 turbo
(so who said you can't turbo a 914
) mated up to a cable shift boxster transmission. the car is absolutely awesome in what had to be done and i think is the epitome of the craft of engineering and mechanics. i don't know how much difference (well, besides diesel and gas) but i would think his project is a great reference to yours. a TDI sounds incredibly interesting re the Audi racing diesel (the one the Porsche RS Spyder races with)