I pull a 5000lb trailer alot. I added some air bags to assist the leaf springs when the back is loaded. I've had zero problems. The air bags keep the stance level with as much as 1500lb in the bed.
Lq9 will certainly be cheaper than an LS1. The extra couple of cubes will help with the trailer also.
No more trucks, but this is what I used to have. 2000 GMC Sierra, 6.0L-LQ4, Built 4L60, LS6 Cam, The Other Guys Headers, 3 inch exhaust and 7 chamber flowmaster, 43lb injectors, Nelson Tuning, Drop - 3/4 McGaughys Spindles, Bell Tech Shackles & Hangers. Best time 13.59 @ 106