I don't think it is too dificult. My wife's 1966 Spitfire had a 1500 engine and the three rail transmission installed by a previous owner. There were no changes to the 1966 frame; it fit fine. We upgraded to a later single rail transmission with overdrive. Since it was an overdrive transmission I had to find a proper driveshft and transmission mount. The Roadster Factory had the parts.
In any case, go for it.
T. T.