I hope this will help, I have a 67 passenger skoolie (66 children @ 3 per seat + 1 driver) and I have 28' 3"L x 7' 2"W of useable floor space behind the driver's seat.
I have 11 passenger windows per side behind the driver's seat that measure 28-3/4" center post to center post. Based on this, a 77 passenger bus would have 2 additional windows per side for an additional 4' 9" giving you a total useable length of 33'. Maybe this will give you something to play with on your CAD program!
I know I had a fun time CAD-ing out mine, & even though I have pretty much decided on my configuration, I still play around with it! Have fun!
~(G)Q Arduously Avoiding Assimilation