Welcome, Dean to the forum!

I am used to hearing about people converting to RHD but LHD almost never! lol

Given that this car will be quite unique where you are gong I would be inclined to go down the turbo 6 route. The F6 Tornado's are becoming pretty thin on the ground now while the XR6T Utes are much more accessible. For me personally, it would be the F6 Tornado for the win in either BA/BF or FG form. The auto suits the T6 best for these cars but the 6-speed manual is more fun.

I suspect the T6 would be much easier to convert. The V8 with is huge quad-cam heads consume all the space in the engine bay. Many people choose to drop the engine out just to fit headers! I suspect relocating brake booster, steering assembly etc will be very challenging, not to mention the engine plenum and induction assembly will need to be reserved. It's going to be quite a job.

The BA/BF 260 and 290 5.4 BOSS V8's area good solid motor that goes reasonably well in the mid to upper range. Not as rapid or as nice to drive as the T6's, however, stock vs stock. If you really want them to sing adding boost usually works best but you are better off rebuilding the bottom end to suit. The Boss 260 in the XR's is a slightly lower risk due to its lower compression.

I hope that information helps. I have a BA V8 and a FG Turbo 6. I love them both!