Re: 351 or turbo250 crossflow
Much easier to get a 351 into an XD/XE than a turbo 6.
I'd actually put a straight LPG 351 into one (well, come to think of it... i DID!) because then you get the best of both worlds, easy rego, no engineers certificate required either.
But if you go the turbo option, even on gas you'll most likely need an engineers certificate, pollution tests (up to $2000) and the cost of a good turbo setup is almost the same as a 351 built for the same power!
Whichever way you go don't forget to set aside a wad of cash for a suitable drivetrain.... stock will die really quick behind either engine!
Just like a Tsunami.