But...my question is: Why would anyone put a Rapide engine in an Indian frame? Makes no sense to me. Why not preserve the original Vincent?
To me, it would be like dropping a Velo Thruxton engine in an AJS frame. You could do it, but why on earth would you?
To gain the suspension and mass of the Indian [and improved Indian double seat of the period] for a road tourer, rather than an around-town or off-road bike. I've nothing at all against an original Rapide, and have about a third of a 1948 Rapide B-series collected and assembled, part by part. But I'd like to continue riding before that probable decade-long project is completed, and it wouldn't bother me a bit to have an extra Vincent motor available as a spare should something go wrong with the original. I don't believe I'd put a Thruxton into an AJay frame, but there are those who've done so with the 750cc Norton engines pulled from TriTon and NorVin conversions. But the velo desmos are very nicely at home in their own chassis, like the Vincent needing only better brakes to rollright into the XXIst Century, and possibly beyond.