The 727 between my 258 and Dana 300 is 23" long, from front face of bellhousing to rear face of adapter, if that helps you. Don't forget the front driveshaft clearance problem the 727 will give you though. 999 won't have that problem.
Going off Advance Adapters spec SBC is 24 inches
the th350 is 21.5" Th400 24.24
and I'm the the transfer case is either Dana 300 or Dana 20
plus you have the adapter itself.....I've just about finished my 75 cj5 350 th350 and even now I'm having problems with tailshaft length...good luck if you try with the TH400