Sorry to get (further) off topic, but I have no information on 304s, so what the hell..
I've gotta say, if I was going to do any hack engine conversion to a track Gemini for great power on a budget, I'd go RB30ET. The response and outright power is phenomenal even for the SOHC, the engines are strong and cost $100 to replace, and there's heaps of parts out there. I don't think they'd be stupidly heavy either. Granted, a straight six is a lot to fit into a Gemini engine bay, but it can be done, especially if you're creative with radiators and intercooling. Even stock NA engines should be good for 100rwkw with a tonne of torque, but slap a turbo on there and you're talking real power. I'd have an RB30ET to drift over an RB25DET, personally.
My mate Justin Codr drives a white Ceffy in ADGP, he has an unopened RB30E+T making over 300rwkw, it's been pretty reliable and goes like a bat out of hell. He can exit tight corners at 2000rpm, and hold the same gear right through to 7000rpm into fast sweepers (ie Tailem Bend). I got to drive it at Mallala and was very impressed.. it's in one of my video edits, hold on...
https://www.youtube.com/watch?v=OoJ8A0BClPk
(Starts with Geminats and Piazza-ing. Ceffy drive is at 5:14, apologies for s*#t driving, it was my first lap and I didn't want to bin it...)