Well, this should get you started:
36x30x20 = 93.5 gallons
Saltwater @1.025 = ~8.5 lb per gal. = 794.75lb
Add liverock, livesand, etc. (minus displacement) = 900-1,000 lb.
This does not include the sump, of course (not mentioned)
After stand/canopy, sump, equipment, etc. I'd say you are approaching 1,500 lb.
I'm not sure about the load capacity of modern floor joists (especially the laminant I-beams that are used now days)
but I do know that, at the very least, you'll want to:
1) position tank/stand immediately adjacent a load-bearing wall.
2) position tank/stand so that they are stradling three full floor joists (they are spaced 16" on center)
3) construct a "pad" to place under stand to distribute weight over entire 36x30 area
4) consider plumbing sump elsewhere if at all possible (to lessen weight on same foot-print as display tank)
If you were planning a longer/narrower tank, then I know you'd be alright, but that deep of a tank places alot of weight upon only 3 beams....that's what concerns me.
I know this doesn't directly answer your question, but it should give you a head start
