I'm running Ultra-bell, JW Ultra-case, the Mizier billet flexplate and JW adapter. No clearance issues at all, plenty of room in the tunnel.

Not sure why it would matter if it was a small or big block, if anything the SB should have more room.

Just had to modify the rear tranny mount that it wasn't any big deal.

It does leave a little softer but if your bracket racing it doesn't matter, I'd just assume loose a little ET for the consistency and since it leaves a little softer you'll have less problems at marginal tracks.