How do they make the 400? Is it the same bore, just destroked? You might be able to build a 400 motor with just the crank, rods, and pistons if that's the case. It'd be a lot cheaper to send those overseas and put them in a 500 block yourself. If the block or heads are different though, that's moot.
Charles.
Charles/Murry
I can't speak to the stroke, but the bore is definitely smaller by a few Millimeters as the Head Gaskets show. The Heads are different as well, with smaller valves. All in all I think it would be prohibitive to try and modify a 500 to match a 400's specs.
I have wondered, however, about boring out the 400 to 500 spec's and replacing the relevant parts to make a 500 out of my 400. Would be nice if it were possible but Carbs, Heads and all would add up pretty fast. That said, the few 500's I've seen here are in the USD $3000 range, ouch. AND the gov here recent outlawed the import of used card and bike parts, they say its to prevent unsafe vehicles, but it's really to protect the auto and bike manufacturers here.
The upshot for me, really, is that on a shoe string budget, and gas at USD $6.00 or so, the loss of mpg/kmpg is not really worth the added cost. Besides which, I've got Larry built carbs and I am very happy with them.
BUT, that's just me. I'm sure there are others out there with 400's who would welcome Murry's Mikuni carbs and be able to fine tune them and work with Murry far better than I can. Shep in Europe for instance or those great folks in Australia or New Zealand.
Mark in Thailand