Re: '91 E-350 tyre pressure.
Just go by what the tire says on the sidewall. Generally, you'll have 'LT' spec tires with either a C,D or E weight rating, which designates the amount of steel belts and thus, the overall strength. A stronger built tire can handle higher pressures, most E rated tires can run up to 80psi. Again, check the sidewall.
Unless you plan to tow or haul at maximum weight rating, you'll find that by sticking with 40-50psi will do you fine as far as ride comfort goes, even if the tires have a max pressure of 80psi, you don't have to run it high all the time. You shouldn't see any extra treadwear from the lower pressure with an LT tire, as they're not rated on the UTQG scale.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!