as much work as you've done to your front end it should be tight
new king pins, springs, ps box and disk brakes as i recall
check all your fasteners and drag link/tie rod ends to make sure you haven't missed anything
straight axle does not handle like rack and pinion your probably used to
but there shouldn't be play in all you've done to it
as for the temp gauge; the needle point to an arbitrary point. mine is overheating to the right of center
yours might go farther to the right, once you figure out where operating temp is keep an eye on it
if your heater core hose is cool to touch then the valve is off
i don't see a charging scheme on the oldair site, vintage air recommends 1.8 lbs.
i tried to use ac gauges with disastrous results, requiring a new $179 compressor.
a friend has a charge machine, now i take it there. works perfect every time.
brakes: these trucks are light, power brakes should lock up all 4 wheels with a quick jab of the pedal.
give the brakes a couple of rolling brake tests and re-bleed them.
maybe an air bubble will break loose when as you work the brakes hard
me thinks your just a little afraid of the old beast
recheck torque, bleed the brakes and then beat the snot out of it