Ha! Trying to get a 16 ga. 3 wire AC cord in a vintage Fender chassis is a challenge. You see this all the time when changing out the old 2 wire cord (and removing the death cap) from old amps. Some guys drill out the hole in the vintage chassis, but that’s not cool if you want to keep it stock. Some guys put a grommet in the chassis and simply clamp the cord with zip-ties on each side, but it’s not a very good strain relief. I switched from 16 ga. to 18 ga. 3 wire power cords a few years ago and never looked back, they fit perfectly. Like this one: https://www.cedist.com/products/S-W123
But, after confirming that the strain relief you have will actually fit in the chassis hole, you can try clamping the strain relief onto the cord, compressing it down onto the cord with vice grips or pliers for a few minutes. Then, push the cord and strain relief through the hole while the whole thing is still smashed together. I use needle-nose pliers to hold it together while working it into the hole.
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