K10kole,
I just dropped a 454 into my 84 K10. I used my original SB clamshells WITH the support plates behind them. I had to modify (drill an 1 1/4" hole) in the driver's side plate because of differences in the blocks and I plan to install the support bars. The BB has a freeze plug in the middle that the SB doesn't. You can shim the clamshells or get BB clamshells. I also used Energy Suspension urethane frame mounts with new aftermarket frame brackets. Everything fit just fine. I don't have the front sheet metal on yet but I don't anticipate a fan shroud issue.