Since your truck is one of the more common ones the better aftermarket fabricators (e.g. Heidt's) will typically have dimensions based on factory holes and crossmember locations. Sometimes they need to be marked before the original crossmember is cut out (if necessary). Another key is if the original has "bump stops", you know, the rubber, bullet shaped devices that mount to the bottom of the rail over the axle to keep it from bottoming out against the rail. These bump stops are almost always at the axle center line. All that being said, it's still best to mock up the new suspension before welding and trial fit the front sheet metal in it's final (meaning adjusted to the cab, good hood edge gaps, square side to side, etc.) position, with the final choice of wheels and tires. That way you can roll it outside, stand back a good ways, and eyeball it for proper "center" appearance (not necessarily an exact dimensional thing). Same for the rear end. I realize that doesn't fit your plan, but if you want to avoid a potential, and major, disappointment later you'll be better off doing the extra work. That's the difference between a hack rod and a hot rod, or street rod, whatever.

And you don't need to make excuses for wanting to put a MII setup in. It started thirty years ago in the finest of hot rod spirit, so I think that's long enough to qualify as a tradition. Real hot rodding is about improving the performance (which includes suspension not just horsepower) as well as the appearance, not just about pretending to know what you're talking about. Another fallacy that will sometimes be thrown your way will be about the MII being too "light" of a suspension for your car. The uniformed (and apparently too lazy to do the reasearch) types who spew that line don't have a clue. The MII's weighed between 2650 to 3100 pounds depending on engine and options. Your truck probably has a curb weight somewhere toward the lower end of that range. That coupled with the fact that the engine in the original MII arrangement was nearly centered over the axle centerline, whereas in your truck the engine will likely be almost entirely behind the centerline. That means that a lower percentage of the total vehicle weight is on the front suspension as the weight center is further back. This also means you don't need the heaviest (weight bearing) springs from the MII either (another popular misconception). Regardless of the weight factors, your plans for additional h.p. would demand you go to a lager brake rotor and caliper than the stock 9" MII. Don't scrimp, and don't con yourself with the "afford it" excuse. If you can't afford to do it right, save up til you can.