Insulation between the interior of the cab and the engine is more than just helpful. If you're planning on cooling the interior it would be smart to cover every inch of the firewall with something that will insulate. In my experience, it would be hard to over insulate anywhere(within reason), not only for temps but for sound as well.
I'm not sure what you are dealing with here but someone will be able to give you an answer.
Remember pictures are always helpful with seeking advice.

~Jim