Sounds like it has a special towing package with the heaver frame and larger radiator. I would also check the radiator cap if it is loosing water with no sign of leaks anywhere else.

 

I did it when I upgraded to the 3G alternator. Best money you can spend.

 

Since you purchased the truck, have you de-coded the VIN ? The reason I ask is... the 351C was discontinued in 1974. The 351M however, was used in trucks until 1982. They are two different engines. If it is indeed a 351C, someone went to a lot of trouble swapping in that engine.

 

The Colorado Edition like others such as the Texas Edition were dealer add-on packages in aesthetics only. What I believe you have is what was considered to be a "heavy half ton" as the F100 was still being produced (until 1984 when the F100 was dropped and the F150 became the only 1/2 ton). The F100 and the F150 are 1/2 ton framed trucks, NOT 3/4 ton. NO "Edition" trucks had any engine/mechanical modifications from the factory. The larger radiator and other details such as additional leaves in the rear springs, etc., were due to it being an F150 vs. the F100. That would account for the previous mentioned comments about it possibly having a "tow package". Unless the previous owner(s) HEAVILY modified your truck by switching over the cab and bed to a 3/4 ton chassis and took great pains to swap in a 351 Cleveland engine along with the transmission, I believe you have a 1981 F150 with a 351M. Still, it's a good thing because you have a heavy half.
I don't want to come across as being gruff, I would just hate to see you having issues trying to chase down parts for a 351C because you may have been mis-informed by the seller.