The only difference between the SBC version and the BBC version is the amount of drop in the center, otherwise they bolt up the same. We were actually using the SBC version for all of them but some BBC applications were hitting the oilpan on the crossmember so we changed them around and added about an inch of drop. The amount of clearance seems to really vary and we're not totally sure why. One example is Fox's truggy with the stock SBC, with our mounts and crossmember it has a lot of room, other smallblocks are way closer. I know in my case with the prototype on Wally, I built the crossmember under my 383 and the 468 fit right in it, no problem.
In general, if you want the BBC later, get the BBC crossmember now and you'll just have more room between the motor and pan. In some extreme suspension cases, you have to watch the diff clearance to the crossmember so if you're going for a short ride height and a lot of up travel, you might want to run the SBC version and if your BBC pan hits, space the motormounts up a little. By "extreme suspension" I mean something like our coilover stuff with a 4" lift and over 7" of up travel with a D60 pumpkin.
Also, the clearance isn't critical for our crossover steering kits, we bend the draglink the way we do to help clear the factory crossmember and typically it will clear without any cutting or grinding. The crossmember is a good strength upgrade and that's the biggest benefit for most trucks. Especially when the factory crossmember is already cracked on the pass. side. This type of crossmember is essential with some suspension or steering systems.
On motor mounts, our crossmember is built to work with the factory type mounts or ours, so either is fine. They leave the whole factory frame box intact and un-moved so it's fine either way. Typically our stiffer mounts will put the motor a little higher and can help pan clearance if that's an issue.
