TH350 with 1.0 3rd gear, 38" tire, 4.37 rear gear, 2500 rpm's = 64 mph.
TH350 with 1.0 3rd gear, 28" tire, 3.25 rear gear, 2500 rpm's = 64 mph.
TH350 with 1.0 3rd gear, 38" tire, 5.71 rear gear, 3250 rpm's = 64 mph
TH350 with 1.0 3rd gear, 28" tire, 4.18 rear gear, 3250 rpm's = 64 mph
If you used a 28" tire and a 3.25 rear gear, you would make the same speed at the same rpm's that you would if you used a 4.37 rear gear and a 38" tire. Now, if your truck did have 28" tires and a 3.25 gear, you certainly wouldn't install a "big" cam and expect it to perform correctly. You would add more gear. Let's say that you changed out the 3.25 cogs for a set of 4.18 cogs in order to "get up on the cam". I'm fully aware that there are no such cogs available for your differentials, I'm just crunchin' numbers for you to show you that you will need more gear with those meats. In order to have the same speed and rpm as you would with 28" tires and a 4.18 gear, you will need a set of 5.71's front and rear. Yes, that is a real gearset for a Toyota differential.
You don't have enough gear to run a "big" cam.