Chevrolet Express/ GMC Savanna

I have a 2001 Chevy Express 1500 with the 5.0L V8 and 135 inch wheelbase.

The 5.0 makes 220-hp @ 4,800 rpm & 280 lbs.-ft. @ 2,800 rpm
It has a 3.42 axle and the stated towing capacity is 5100lb
the trans ratios are:

1st-3.06
2nd-1.63
3rd-1.00
4th-0.70 (OD lockout is recomended for towing)

Chevrolet reccomends brakes over 1000lb and distributing hitch with sway control over 4000lb.
I have pulled our 12x6 enclosed cargo trailer(3000lb) with no brakes without any problem.

I'm looking at either the 3124ks or the 3326 king. Would these be within the capacity of the van? Also I may replace the van with a 2004 with the 5.3 V8 @295hp/335lbft which puts the towing capacity upto 6200lb.