Radiator/heat issue.

Just bought a 91 mighty max, and was running some strong heat. Went through it and changed all the fluids. The radiator is brand new, and was quite dry. Now that I went through it. Not much heat, and engine is running cool. Good for the engine, but uncomfortable with all the brutal cold we have had in New England so of late. Do I now dig into the thermostat and change it? I was wondering where is it, and are there different ones that open later for a warmer running engine, and better cab temperature? Any help will be appreciated. Thank you Gs.

If you get really cold winters I would recommend changing the thermostat over to a winter temp unit. I have been a big advocate for deleting the pump driven cooling fan for an electric thermo fan as it is way more efficient at maintaining engine temperatures in the heat - but it also is beneficial during cold weather as well. The mechanical fan keeps going until the clutch releases from ambient heat (which isn't going to happen in a hurry in near freezing temps) while the electric fan won't kick in until the engine is in an optimal heat range. It'll let your engine warm up faster and will allow the heater core to circulate coolant hot enough to make cabin temps bearable.

also might be a good idea to make sure all the air is out of the system.

It would've probably helped if you were told where the thermostat is located (my bad). On the inlet manifold where the top radiator is connected to. It's held down by 2 x 12mm bolts. Might pay you to give them a bit of a soak in WD40 before attempting to remove them as they can be bound up in the threads with rust and strip them out. If you get really cold weather you can cover part of the radiator with cardboard or corflute to restrict air flow through it. I remember one winter we had here the air temp before sunrise was so cold it was causing my auto choke to close by itself. I had to pull over and let the engine warm up before I could drive again... Jammed some cardboard over part of the radiator and I was back motoring my way to work.