This is a hydroboost conversion from a vacuum booster?
I'd expect you need the pedal from a GMT400 hydroboost vehicle.
But this is not a conversion I'm familiar with. I have considered it for my '88 K1500, though.
From his signature (which not all will see) he's got a 1500 but yes, it's always helpful when posters give us more details. It'll be a conversion on his truck.
OP you'll want a pedal from a '95-up hydroboost-equipped truck. Go to the boneyard and look for 3500's, as they should have hydroboost regardless of engine type. The 2500's it may be hit or miss, but the diesels will have it for sure.
If you're unsure on the year model, 10th VIN digit "S" is 1995, so anything S or after. If it's "R" (1994) or below, it'll be too old. It should be obvious by the interior style but depending on how stripped out the truck is, you may not have many other clues besides the VIN. The older model truck pedals *might* be the same but I'm not sure; maybe another member knows.
Richard