sundown
+1y
i will reiterate what was said above, measure measure measure, but all in all it is pretty easy so dont get discouraged. What I did on my 84 was mark the centerline of the axle (using a plumb bob) on the framerails, i marked on botht eh inside and outde of the framerails. I then measured (just to be sure both sides were the same)from the marked centerline on the frame diagonally to the front crossmember. Both sides just to make sure they were pretty close.
then measure the inside width of the notch plates. I dont remember how wide mine were but lets say they are 5 inches. mark a line on the plates at 3" since that is the center of the notch.
to find out how much notch I wanted to use I did a little math and measuring, I hate both but you have to do it to get this right.
I measured the distance between the bottom of the frame rail and the ground, then measure the distance between the bottom of the frame rail and the top of the axle. subtract the two and what you end up with is a number in inches that you need to allow for (or notch) with your current wheel and tire. I used diffrent wheels from stock and plan on going bigger so I took that into account. Remember that since your axle will be passing up through your stock framerails to take the height of the frame rail into account for some of the inches you need to notch. If i remember right the frame rails were around 3-4" tall, so I used about 5" of my notch.
Anyway, at this point you should know how much notch you are going to use so put the two side pieces on, align the center marks you made, make sure you have enough height on the notch to do what your looking for and start welding the sides on the stock frame rail.
Once the sides are on, you can measure what you'll need for the top of the notch, cut it and weld it in place. I let all my welds cool completly before the next step (that was just me though). Next take out the sawzall and cut the stock frame rails from your new notch. once these are out of the way you can measure for the underside boxing portion of the notch, cut it to fit and weld it into place. Be sure to cover the ends of the stcok cut framerail with steel as well and weld those shut.
Take your time and put down some good beads, this doesnt take long to do (took me a day and Im slow) but take your time and it will turn out fine. Also read that above link to s10forum, it will give you good insight as to what needs to be done too.
good luck and lets see some pics when its done.