Yes, you should be able to neck down the 338 Excalibur brass to 300 Pegasus.
But...You may run into a neck clearance issue depending on the reamer used to cut your chamber, as the neck wall thickness may increase up to .002 to .003.
Can you measure the diameter of the case neck on one of your fired 300 Peg cases?
Can you measure the diameter of a case neck of a loaded 300 Peg case?
Post those two dimensions....and we'll see if this has a good chance at working in your gun....without having to neck turn the 338 Excalibur cases.
