Got the underpowered (for my AS) 5.3L in a Suburban with a 28W -- we manage OK hereabouts. If you are in a hurry (>60MPH), it is not the thang for you...
My stepson has the same thing (5.3L) in a Tahoe, and I could (next time he is in town) hitch it up and see what it is like towing the 28W. Not sure if he will be back soon, though. I remember a line from that classic movie "I am not sure if I was pulling it, or it was pushing me." The Suburban handles the uneven pavement, crosswinds, and close quarters with 18-wheelers in constructions zones without too many scary moments. Overinflating the rear tires a bit has help reduce sway to nearly zero, when it was pretty small before...
Is there anything from the 70's that you can put a modern engine and tranny into that could meet your requirements?