Transmission for a rear mid-engine RWD car?

I'm looking to fabricate a '67-68 inspired Formula 1 car. Budget is a huge factor. I can't seem to find definitive info on rear mid-engine transmissions that I can mate to a small block Chevy 350 via adapter plate. I've decided on that particular engine for availability, simplicity, cost, and availability of support. So, what is the cheapest transmission I can mate to a SBC 350 that can handle 250-300 HP? Most of the Porsche transmissions cost more than the total price for all other materials for the build. I've only been able to find rumors and ideas on what you could POTENTIALLY mate to them, but nothing on anyone actually doing it.