I do have a soft spot for a 3i and that one wants getting of that damp floor ASAP.

5k sounds a lot and is a lot but it won’t do a fully forged reliable 300 with room for more.

To start with the gearbox will shit itself with that much torque. You will be looking at an mtx75 conversion. Probably an Lsd. That will swallow a grand with out a decent clutch.

Engine management.with a couple of dyno sessions will sink another £1500

custom manifolds, decent turbo the project will really escalate.