From research I've done on the internet, there appears to be a big crossover of parts between the various Suzuki twin engines. I'd initially thought of using the GS450 frame because it's a duplex frame like the old BSAs, but the exhaust ports on the GS500 face exactly forward, they're not splayed, and as such the header pipes would foul the frame.
I looked at the GS250 frame, which had the single down tube and the GS250 crankcases looked the same size as the GS500, just different mountings. The Eric Cheney frames used a single down tube so it must be ok (ignoring differences in steel type and diameters). Funny thing was, the guy that I bought the GS250 frame from had a project bike with a GS500 engine stuffed into a GS450 frame, but the header pipes were really, really tight with some tight turns to avoid the down tubes.
So the GS500 engine does fit in the GS250 frame, just, with new mountings welded in. However, to adjust valve clearances will involve removing two mounting bolts and tilting the engine forward to allow the cylinder head cover to be removed. But as these are checked every 6,000 miles (I think), it won't happen too frequently for a trials bike. Say once every 5 or 6 years?
As regards twin or single carbs, my belief is that a single carb is better for bottom end torque and tractability. Andrew Berry is running his 750 Wasp outfit with a single carb and that outfit has no shortage of power. Twin carbs are better for the top end power where the engine would otherwise run out of breath. Aside from all that, twin carbs won't fit with the GS250 frame, the seat tubes are in the way of the inlet tracts. It'll also make constructing an air box that much simpler to have a single carb.
I'll post some photos when the bike looks in better shape and is accessible. It's buried under piles of crap at the moment as the garage is a bit chocker block
