Right, you want two flat bits of steel, then just cut and bend the ends to lip over where the front bolts go through. You then drill a couple of holes in it (blue) to mount the side mounts (you'll might want different ones that have more flat metal on the base to be able to drill holes exactly where you need them), and then you can have the seat right up against the tunnel if you want. Or you can drill new holes in the runners if you want to fit those, just going to take a bit of fiddling to get to where you want.
I would be very careful about using any kind of spacer tube, as you make the bolts much more likely to sheer off like that if you had an accident. Keeping every flat and bolted together is far preferable.
To my eyes though it looks like those seats and that frame won't ever be compatible, but without actually looking at it in the flesh (or a video?) it's hard to say for certain. One question though, does the outside holes where the frame bolts down match the inside holes in terms of height? I can't remember if it does on a 350, as if it does this is all for nothing as you'd need to space everything up again which we're trying to avoid...