I have a 96 GMC 'burb with 350 and rear air and heat. It seems like it takes a really long time to warm up. I did replace the radiator with a 3 core, biggest that would fit, last summer. I assumed for a long time that this was the reason and the extended water system to run the rear heat, but it seems to take a really long time to warm up. I drive 8-10 miles to work and the temp stays between 100-120. My other 99 tahoe was slow to warm too, but I thought I would check with yall and see what you think.

I had a Silverado with the 350, that sucker would take forever to heat up. It's easy enough to pull a thermostat and test it. Might as well replace it with a fail-safe one.

Sounds like it is stuck open or have the wrong one in it. Probably ought to check it.

Replace the thermostat with a factory 195 thermostat...after you replace it you will hear the heater core gurgle for a week are longer until all of the air is out of the system.

Replace it. Thermostats are cheap but can cost you big bucks.

You say you put in the biggest radiator you could find right? I did the same thing to my last truck and I had to change the thermostat back to a 195 to get any heat out of it in the winter. It had a 180 in it when I was having an overheat problem that turned out to be a plugged up radiator. The only time it was an issue was in the winter. The short drive to work is another issue. With that big radiator, it doesn�t have time to warm up at all

98 suburban 3/4 ton with 454 just had same problem.
new radiator and just would not get hot.
pulled factory 195 stat out,looked fine,was not stuck open.
put new one in and warmed up to 195 in about 3 miles.
run the engine with the radiator cap off for a few minutes to purge the air out.