So I'm thinking of upgrading the 3.0 to a 3.5 but the current motor has gas conversion. Would anything need to be done to the new motor beforehand or is it just a simple matter of swapping it over?

Moved to correct forum...

Try sending Tyson (T_double_U) a PM, he has a gas conversion on his 6G74 and know quite a lot about these.

Cheers :)