the shroud shouldn't be a problem while the electric fans are running.
look at a circle track Late Model. they cool +700hp v8 engines through nose openings ~ 2" x 20". unless the previous owner completely shut off the nose, you've likely got enough air flow.
the T-stat obviously needs to be lower than the trip point for the fans. and there's no reason for +185 anyways unless you're in the far north and need the cab heat.
have you checked the lower rad hose to make sure it's got the support spring? that's the vacuum side of the radiator and, if not supported, can get sucked shut.
you can get a coolant fluid test kit that will tell you if you have exhaust gases in the coolant. if yes, there is ( probably ) a head gasket problem. most decent mechanics will also have this.
check condition of the water pump. the pump housing is aluminum and the block is iron, the dissimilar metals can cause electrolysis damage to the aluminum housing.
the 71 manual will have trouble shooting for the 455.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)