Sand is a lot like snow, you should air down to below 10psi, probably closer to 5 psi. You want as alarge of a contact patch you can get, so the lower the psi you can go, the better traction you can achieve.

Even if you do shoe-horn in a 351, you would still need to lower the tire pressure.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed