LOL I'm thinking about putting Flat Out Engineering Corvette IRS under it. I used their kit under my 55 pickup. I don't think this is that much heavier.
Where you mount the engine will make a difference. My SBC is under the bed. My frame and bed are HEAVY too, but I still only use 1/2 ton springs up front. Anything more is way too stiff.
I don't have a weight on my Autocar cab, it tips easily with a bumper jack, but more than I can lift by hand, so somewhere in the 6-800 lb range. (with the attached sleeper) I have just over 4000# on the front axle and still need to add an additional 1000# of batteries, interior, transmission, and other doo-dads before it is all done. I'll probably be between 8 and 9000# when finished.