I want to say that you can use a Trapezoidal belt on a Curvilinear gear, but you can't use a Curvilinear belt on a Trapezoidal gear. As for Mitsu having the wrong cam gear. Have you owned the truck from the dealer when it was new off the lot? If not who knows what may have happened over the last 28 years. New head, truck was crashed and broke the cam gear, who knows?????? :{? It might be worth looking at the other belt and checking to see if it is correct. My self I would want all the parts to match. round to round, square to square. But thats just me. Might be worth looking at the junk yards to find the correct cam gear. Ya never know...

Do you have the old belts laying around? Get us a picture.. We like pictures. It helps the next guy avoid problems.
If you still have the old old belt check the cog shape.