A 'known good' temp guage or maybe one of those infrared temp guns- My mechanic has one and he points it to the running engine to find out how hot it is (always hotter on the therm housing).

Heater core would cause problems if it was leaking, with the heat off some heating systems won't let water to the heater core, so I'm not sure that if it doesn't leak it could be a source of the problems.

Don't think your truck came with a shroud, but one could be added to improve cooling.

Another area might be the 'fan clutch' if you've got one. When those go sometimes the fan doesn't spin at all.

I always start by checking to see if the system is full and not airlocked. Then I check the water pump. If there is no leak at the pump and no way the hole is plugged with grease or excess silicone, then its on to the thermostat. Once that is changed, if it is still overheating. Its gotta be the radiator- Its easy to pull it and take it to a radiator shop to have it cleaned. Often you can have it recored for about $100. I'd check with Autozone onlinet (or whatever other FLAPS is close to you) to see how much the radiator is and if they can get it 'cause it might be cheaper to replace than recore if the recore gets up into the $150 range.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi