It can be done
 

I changed the fuel pressure regulator on my 89 7.5L without removing the upper manifold and fuel rail. It wasn't exactly easy getting those three screws out and back in but compared to taking half the engine apart it was easier. Now if those screws wouldn't have been Allen screws it may have been a different story.

I don't know if the refulator has solved my fuel consumption issue as I haven't put any gas in it yet... still on propane but I'll need to run gas when it gets cold and for about the cost of a tank of gas a new regulator can't hurt.