I think the 4L80e is longer than the 4L60e - but - the 700r4 has come a long way in durability and ability to hold pretty amazing power levels; thus, do you really need the 80e?
My first thought was Cadillac motor and turbo 400, but that'd be so overkill for what I do - plus, the 500 ci cadillac motor is a wide beast...
I'm building either a 400 or a 350 for mine - in any case, I'm moving the motor as close to the radiator as possible, and I might add 2" in front of the rear wheels to get just a bit more... dunno yet, I really need to start putting large bits in place so I can visualize what's next.
As for that transmission. My daily driver is a H3 Hummer Alpha - so I have a 5.3l LS motor, 4L60e transmission, 4:1 transfer case and e-lockers front and rear. The truck weighs a donut more than 6000 lbs with me and my SAR gear in it - and I am not nice to it, it came to me as something that never climbed anything higher than a curb at Nordstroms; so I feel obliged to beat the south out of it every time I play. I've broken a front differential, the transfer case (a link came off the chain and did no good to the gears it got caught between) - but the 4L60e hasn't even whimpered and I've put 40k more miles on it from when I bought it at 80k.