i'm doing an essex v6 (mustang engine) swap into a first gen right now... i'm using it with an AOD trans. the height and width of my v6 is almost identical to a 5.0, so what i've learned will apply to you also.
first i've learned that you need to enlarge the tunnel a little for the trans..
second, i hadto remove the heater box entirely (valve cover is sitting right in the middle of where it used to be) i also had to remove the a/c compressor from the mustang engine. on a 5.0 you're oil filter may conflict with placement of the steering box so you're probably need an oil filter relation kit. also i'm moving down the X member 4 inches to make room for the oil pan (this will also move down the axle pivot's 4 inches which is required for lift i'm doing anyway)
it's not an easy swap at all, and with the added length of the v8, you may need to be creative with you're radiator (move it forward where the a/c rad was or something)
it's a big pain, i've been working on it for about 4 months now, i would consider myself a fairly experienced fabricator/builder, and i have a friend (who has a lift and is very good with metal fabrication) helping me with it.. it's coming along, but very slowly, and it's turned out to be a ton of work with many complications. i also had to relocate the battery.
as for you're original question, i suspect if you use aluminum heads/intake, etc on you're v8 it will weigh similar to the v6. you may need to adjust camber slightly but i wouldn't expect it'd be far off.
just curious, why do you keep blowing 4.0's? my '93 has the 4.0 with 220k miles on it.. and i'm not easy on it at all.. i tow an 18' flatbed trailer with it, and have towed other trucks on that trailer regularly.. and i'm not afraid to floor it up hills (at near redline rpm) i dont mean sissy floorin it either, i mean pedal hard against the floor, for as long as it takes to get up the grade (several minutes, half hour, whatever). i couldn't be harder on it if i tried, and it's been fine.. what did you do to blow yours?