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Sure it can tow 6000# but dont worry about trying to use the highest gear. RPMS are your friend.
Just waiting on you to postI thought you were gonna chime in and extoll the virtues of boost when I saw you'd responded.
Just waiting on you to post
Now a 5.3 with the right size turbo will tow like 6000# isnt behind you.
That's why you need an '80.....And gut that 4L60 like a fish!
I use a 2011 Suburban with 5.3/6L80 and this is exactly my experience with it. It does it fine, but it is gutless and sucks gas. It got better when I rebuilt the diffs and regeared to 4.10's, but it is still rough. We tow a 6k lb travel trailer with it and it will cruise at 70mph in 5th gear no problem. Pulling hills in the Cascades or coastal mountains it will drop to 55mph and 3rd gear usually. Part of that is that it is an AFM engine and has 230,000 miles on it. It is definitely hurt. I just recently got a donor 6.2 for it and am in the process of cleaning it up and rebuilding it with a decent towing cam to replace the 5.3, that should make it a much better tow vehicle.I cannot stand the 5.3L even with a 6spd and 3.73s in an unloaded Express van. I test drove one overnight that the last dealership I worked for took on trade and had priced for a steal. It was always shifting around and just plain gutless unless the pedal was on the floor and the engine reving to the moon. I do not think towing would hurt the 5.3L, it would just be gutless doing it. I ended up not buying it because I would have had to put a 6.2L or 408 stroker in it for me to be happy with it. FWIW this was before I built the 383 for my 97.
SBC valve should not be hard to find. What size valve is it?