ERIC - That's odd that the center blade was a bit too long. Dad had a 42" deck that he never used, came with a pristine original condition 129, frt blade he never used, and a rear tiller that he used ALL the time.
I've had several 38" decks, and the center blades on a 38 & 42 are the same, the outer blades are just longer on the 42" deck. MTD must have changed suppliers of the blades and now they run long, or like I've had happen at work dozens of times, the new supplier finally made them to print! My current 38" deck was missing the baffles and I rebuilt them 30 yrs ago higher, thicker, and closer to the ends of the blades along with some other changes to the deck.
It shouldn't hurt anything grinding the ends off as long as you keep the blades in balance. I always balance the blades over the shaft of a round screw driver and make sure the blade balances with the screw driver splitting the center mounting hole perfectly in two. I also wire brush all the dust/dirt off the blades before I sharpen and balance them. I use a twisted wire brush in a drill press to clean them up.
Grass, dust, leaves, etc collect under the belt guards on those older decks and hold moisture if they're left out in the weather. Rust and springs don't mix well. I always blow out under those belt guards when I'm done mowing as well as blow everything else clean on the tractor with compressed air.
The tensioner needs to be pulled towards the front of the tractor when the mower is mounted, the belt installed and the belt set into the idler pulley. When it's right, the belt runs parallel to the right spindle pulley by the discharge chute, the back run goes to the left side outer pulley, and the front run to the idler, then curves forward to the center spindle. The idler has the spring force the idler towards the back of the deck.
The spring very possibly could be twenty years old. Those deck belts normally last a long time. It's the mule drive from the engine down around the two idler pulleys under the engine then back to the center spindle that go bad first. I would say the mower deck belt will last for 5-6 mule drive belts. Think I've broke only 2-3 mower deck belts on my 38" deck in the 31 yrs I've owned it.