You could probably use a 5.0 crank, but youu'd have to have the whole assembly rebalanced. Another added cost that gives you no added power in return.

 

Seriously, you can get a 5.0 for less than $100 these days, the 255 is just that, a boat anchor. I have seen 255 heads on a 5.0 (dont know why anybody would do that) several times at the JY, so the bolt pattern is the same, I suppose you might be able to port a 4bbl intake and make the ports match a little better, but I doubt you would see that much of a difference because the heads are such a bottle neck and the engine wont pump that much air anyway if you cant get the RPM's up, same reason a cam wont do anything.