I'm just going to throw this out there but what about the Ford 3.5 ecoboost? Not sure how it compares in size to the VW V-6 but it seems this would be an ideal motor for the GTA if it would fit. There are tons of them out there, they are relatively cheap, HP and TQ can easily and reliably be in the 500 range not to mention Ford Racing offers them as crate motors.
This the trans people are thinking about?
No...Merim already posted a link on top of the previous page. That's QUATRO and will not fit.
I was thinking of a twin engine setup. Like this one https://www.youtube.com/watch?v=HB5Qp1RcGHI
This Talon was one man's exersise in design and fabrication, to do something most peope said was impossible. And it was great, as such! However, it is a terrible concept for a viable vehicle...
Doing this to an APEX, in its currently proposed configuration, would be no easier than doing it to ...lets say... a Corvette...