[dsraven]

glad you have made your decision. like snickers says, your truck, your decisin, just make it safe, which it sounds like you plan to do.
if fishplating and frame grafting is new to you or anyone lurking, google the topic as there are many ways to do the same thing but some provide a stronger seam and less apt to crack depending on how the fishplate is made and welded in. you could also drop by a heavy truck frame shop and see how they shorten or lengthen van body trucks and do their fish plating as those units are all over the place and you don't see them cracked in half, lol.
I recommend a digital level, even a digital torpedo level set on top of a longer level if needed, as they are going to be more accurate than the old eyeball from different angles against a bubble level. they are pretty cheap and some have a laser built in which can help you line up the front to rear down the middle. what I do is mark each cross member at it's center-side to side- then run the leaser or stringline down the middle to ensure all the parts are lined up before tacking. a plumb bob from the axle center lines with a correspondng mark on the floor and then cross measure is also a good idea. you probably have it figured out though, it sounds like it isn't your first rodeo.
hopefully you will post up some pics as you go, it will be interesting to see how it fits up and how the ride height will turn out.