The vanguard guys prefer the horizontal engines mostly because of the oiling system, and the crank shaft is 1.25" shorter/lighter and they have ball bearings on the pto side compared to the bushing in the pan of the verticals. The other advantage is that you can offset the engines a bit more, building headers isn't as hard and usually there is more room for carburetors too.
You can make any engine work, I don't think there is any more loss of hp through a RAGB than a good jackshaft set up its self, but the chain drive might be a bit more efficient over a hot belt, I'm sure the difference is minimal.
I glad to see more interest in the supermod class.
George your new supermod is a real nice looking machine.