Now this advice comes with the knowledge that we overheated our VW car last year because of a stupid not paying attention issue while building the motor in a hurry. But I have never over heated my Class 9 off road car so....
What kind of air cooled motor are you going to run? I have lots of experience with VW and Porche based Flat motors from racing off-road. My advice would be 50 weight oil, external cooler with electric fan. You should be just fine if you do that. www.race-dezert.com is a great place to see pictrues of other peoples cars and how they set them up for cooling. I would stay away from a corvair simply because of parts availability. Of course Jay and the guys like rare and hard to find cars better than average cars so there may be something there.
If it is a VW based car love to chime in on what we did and I can tell you after Houston what worked and what did not for two differnt years with an Air Cooled Motor.
Organizer's Choice Houston Yee-Haw Its Lemons 08', DNF Houston Yee-Haw Its Lemons 09'
Ugly Class 3 Winner Houston Gator-o-Rama 10', Ugly Class 3 Winner BFE Colorado 10', Ugly Class 3 Winner Texas World Speedway 12, We got Screwed Winner ECR 14', Barely Finished MSR 14' took a few years off but trying like hell for IOE in 2017!