Hello Deereboy, information from both Steve and Dick is spot on. I suspect what Dick was trying to say was that the inside diameter of the rims should be a nominal 21" which should be 20-3/4".
Since it is a chassis and possibly the remains of someone else's project make sure the discs are in fact 28. I am probably going to mess up the dimensions but I think the 25 through 27 discs were 2-1/4" in width, and the 28 were 2-3/4" in width. the 28 discs should be 21-3/4" at the inside flange, and would fit a 21" tire. When looking at rims remember this style of wheel was made in 19", 20", and 21". Be careful on the 21" about the width. The 20" rims are somewhat obvious as to the width between the car rim and the truck rim.
If you need some pictures let us know.
Art