Where to find parts for 1978 Pete 352?

Up until around 2007 the old door handles were still available from Pete. Courtland should have those. And they are the same for all Pete's from 1954-83. They use the old Briggs and Stratton style key that Gm also used in the late 50’s thru ‘67. Even if you find one without a key Locksmiths anywhere can re work them for the keys to match.
You won’t find a replacement pocket step new and chances are the ones in other vintage cabs are just as worn. That really isn’t that bad. I’ve seen worse. Removing the huck bolts and re installing with new hucks isn’t worth the pain when you can fix that with weld. Put a round aluminum rod in the crease/fold on the inside/backside and weld it thru the crack on the outside. Grind and buff the bead smooth and feather it in.