Now for the bad news!
If you try and put anything in the Spitfire with increased levels of Torque over stock say North of 150 Torque you will destroy your rear differential in short order! They were built weak and not designed to handle much more than stock. The GT6 later differentials in the MK3 were stronger but still will not stand up to constant punishment from a torque increased engine. There are modifications you can make to these differentials to net even more increased strength but it will never stand up to a modified high output street or racing engine!
The other issue is the swing spring in these Spits they are dangerous under high acceleration they cause jacking which is not a pleasant feeling when you are driving to say the least. The later GT6 MK2 and MK3 went to a rotoflex suspension and vastly decreased this jacking effect but did not do away with it! You still had it with high output engines over stock!
If you want an engine that is gonna be high output diesel or gas you will need to sort out these 2 key variables as they will be show stoppers for you guaranteed.
My TIGON diesel electric hybrid GT4 model is using a VW 1.9L TDI with a T56 6 speed transmission. Currently I am trying to sort out the rear end as I stated above its not meant for this kind of torque! Many people have swapped in R160,R180 Suby and Nissan diffs with great success. So you might check into that as an option. There is lots of info flying around the net in Triumph circles on this upgrade.
The other thing to also keep in mind is the Spitfires can easily upgrade the breaks from the GT6 models as parts are easily swapped between the 2 cars. If you go with a higher output engine I would highly recommend swapping your brakes for a GT6. They are bigger and will help you cope with the increased power levels better resulting in a safer and more reliable ride.
GH
