[dsraven]

set the truck on a level surface and with wheels etc on. if you have a rake angle picked that the suspension is set up for then get the truck at that angle. long story short, get the truck sitting at the rake angle and suspension ride height it will be run at. whether that be sitting on stands or whatever. just be sure to have the stands under the axles so the suspension is carrying the weight like it will be when driving. next, put an angle finder on the engine oil pan rails and mess around with a floor jack under the rear of the trans untill you get the engine at a 3 deg "down at the rear" angle. install the rear trans mount of your choice, set a level across the trans mount to cross member surface of that mount (level it of course) and hold there with cable ties or whatever. measure up from the level surface to where you want the cross member to be mounted on the frame. that measurement will be the amount of drop member required.
pretty easy now to draw that up on a piece of cardboard from a large box and take it to any welding shop with a pic of the bought cross member for their reference of what you want. they could fab one up in no time. then bolt it into your frame. the shop will need to know the width of the frame rails at the mounting point and how much slip you want for one tube to slide inside the other (for width adjustment as per the bought member)
so 3 deg down on the engine/trans combo and 3 deg up measured on the flat of the pinion yoke if you don't have a drive shaft. d/shaft needs to be the last thing in the puzzle after everything else is finished, mount wise.