any attachments on it? attachments can cause heat build up also.

other than that i can say i worked on quite a few of these trucks at home depot where i live and we had overheating issues too.
The ONLY thing that fixed the problem was to put dexcool coolant in it and also put an additive called enginkool made by CRC.

With the correct mix of dexcool and that enginkool added in the cooling system the trucks would run at a decent temp and not overheat.

You could also cut some extra vents in the kick panel in front of the engine on both sides of it to allow more air flow, that might help some as well.