Since you'll have a spare vehicle on your hands, have you just considered rebuilding one of the L31's?
If not, I'd go with a crate L31 over a 5.3 for sure. I have both, and unless that 99 'burb has 4.10 gearing or more, I think you'll be disappointed in the 5.3 in a heavy vehicle. My L31 pickup with 3.73 gearing walks all over my 5.3 Suburban with 4.10 gearing when towing 5000 lbs.
And a 5.3 or 6.0, no matter how cheap you find one, will nickel and dime you to death by the time you get it in and running.
And there's the unknown history/service life of a used one versus a slam dunk 200k+ miles you'll get out of a new crate motor.