I put a Ford Pinto 2.8L V6, 4 spd in a '72 Spitfire about 10 years ago. It took an extra u-joint in the steering column, but everything else was stock. A 302 would put you in league with Carroll Shelby.
The spitfire already has a decent rack & pinion steering - actually it was a pretty advanced car in 1962. (full independent suspension, disk brakes, 62 HP ...)Don't know where the steering column will interfere on a 90 deg V8.
The Spitfire frame is very narrow where the front suspension locates the upper A-arms. If you stick with the Spitfire front suspension, the exhaust head pipes pipes will have to go over and outside the frame rails, Due to the double wishbone shape of the frame rails there isn't much room down the middle for a transmission, driveshaft, and exhaust. Unfortunately, there are outrigger supports from the rails to the door sills that eat up most of your ground clearance.
Good idea to consider the Pinto/Mustang II front suspension, I'd stick with the Ford rack. Once you have done that up front - and changed to a Datsun IRS - I would look at statrting with a full custom frame in between. Then drop the Triumph body on it. You could probably sell the un-molested, rolling, rust-free Spitfire frame to someone doing a restoration.