I know this costs money but we do it when rebuilding. Take it to a frame shop and make sure its straight. I know people will say how good the parts fit before replacing with new and now they don't fit but many times the new install problems can be fixed with a simple frame check. Its amazing how the problems go away when the alignment is correct. We do this for a living and you want to eliminate as many problems as you can.
If on a budget
Like stated above get a manual and at least check the cab to frame squareness now before bolting it all up. Get it on a concrete floor (clean)
Draw some layout lines on the floor. A chaukline with some clear spray over it works or whatever and start checking dimensions. You can use lasers now to help get the lines straight and perpindicular. Or use the ole 3 5 9 method.