Welcome BabyJKC!!
Nice Stepper you have there. The e350 wheels should work for a future disc change if they are the Accuride 16x7's. Try to look for 73 and up GMC caps in 12" variety; they may be a bit easier to find although not period correct, but close.
Far as adding power steering, first you need to acquire the double groove pulleys; crank and fan. You can stop there and fabricate a power steering pump bracket or continue finding the factory one, another challenge as they too are tough to find.
Here is link to show what the factory type look like. I have seen some use idler pulleys and one with a Big Block chevy-style with remote reservoir.
Again the key is getting those pulleys. Best of luck and again
