You indicate there is a 600 amp main and a calculated load of 525 or 540.
(2) parallel 350 AL is out as that would only give you a 500 amp capacity, too small for your load and your main..
(3) 4/0 AL would give you your 540 amps (unless you placed them all in 1 conduit).
Darn it your right at 70C 350s are rated at 250a each