Which steering box do you have...Bendix or Saginaw? The Bendix box is inherently weak and prone to fail and it is driven by a low-pressure pump. Don't know about the Saginaw PS system. My guess is it might stress the pump and pump bearing unless it is in good shape, and maybe stress a Bendix gear box...more pressure might mean more leaks. The bottoms in the Bendix gear boxes have weak seals. Why are you going to a larger crank pulley?
I have a saginaw steering gear in the truck (which is a 72 F250, 390). Both the steering gear and pump are new (remans).
I put a 3G alt on the truck and am changing over to dual vbelt for the alt. Went to the junk yard today and found the alt, water pump, and crank pulleys I needed; but the power steering pulley is larger than the one I had on there.
Looks like I'm gonna have to locate a 6" pulley and put it on the pump.
Matt, I have a suggestion that might help with no out of pocket cost. Lift the front wheels off the ground, then turn the steering wheel bump to bump about 20 times. This can help purge any air that might be trapped in the system, then give it a test drive to see if there is any improvements. This may not solve your problem but is very low cost.
If you can't find a larger pulley I wouldn't worry about it based on what nevrenuf says...slap it together and run it, and keep lookin for a bigger PS pulley or pump with bigger pulley. The Saginaw boxes are strong customers.