Just a couple of thoughts from an old guy who has been there, done that.
Get a contractor's plumb bob and some chalk that will write on your concrete surface you'll be working on. That's how to do all the measurements, drop the plumb bob from point to point, make a mark on the concrete and measure with a rule of T-square. Get yourself an angle finder and a couple of good levels, a short one and a long one.
Chock the truck up at ride height. Take the front wheels/tires off and put jackstands under the frame at the height the truck will be at when finished. Put the motor in with the intake manifold in place. Put the level on the carb pad of the manifold and level it front to rear and side to side. The motor and trans will be installed at a down angle when you do this, usually 2-3 degrees.
You can move the motor front to rear as far as you want to either way to make things fit. You can also move it side to side either way as far as you need to, so long as you keep the crankshaft centerline perpendicular to the rear axle centerline and watch for transmission clearance at the floorpan tunnel. Don't be afraid to axe some sheetmetal out of the way to make it fit. You can use scraps and weld it back together later. This is particularly true of headers. NOTHING should get in the way of your headers. Axe out anything that is in the way. Never hack up the headers.
Pay attention to service items like mechanical fuel pumps, oil filters, etc. Don't box them in so that you can't get to them to change them later.