With that big hole (the grill) on the front of the tractor, I don't see anyplace for the cooling air to go but out the front or out the bottom. Perhaps it acts as a heat shield to keep the side panels from getting too warm from radiant heat off the muffler? I haven't noticed this to be a problem yet. But like you, I didn't have the aluminum frame, so adding the sheet metal box was not practical.
IH obviously felt like it was helpful to cooling or noise management, or they would not have gone to all the trouble to put it on the tractor to begin with. Since it restricts and channels the air flow forward, it must decrease cooling air flow, even if only slightly. I'd guess by 10%. Probably not material.
The creeper gear was optional on these tractors. It's primary (intended) use is for rototilling and snowblowing, to reduce ground speed. It's commonly thought that it creates a weak point in the driveline, and some have suggested it not be used on a tractor intended for garden turn plowing duty.
You can usually pick one up on ebay for $200-$300 dollars. Repair parts are mostly still available for them. I also put one in my IH-148R project (in the customized cub cadet forum).
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