Mike,
The BB will definitely not fit under the motor box. I know others that have split it down the middle and made it wider, then reupholstered. They turned out very nice.
IMO the BB is highly overrated. The performance just doesn't justify the extra weight and loss of cabin space. A well tuned stock SB with better flowing heads and aluminum intake will out perform a stock BB. Add a cam to that scenario and it isn't even close. My SB stroker will outperform (in all aspects) any stock BB hands down. I know a lot of other guys that will say the same thing. They'll take a SB any day.
Now, if you have a the HO BB (425 HP) or have a modified BB it's a totally different story. Definitely go for it then.
FYI, You'll spend a lot more money modifying a BB too so you have to keep that in mind.