I have two frames spliced together on my truck. I splice it at the rear gas tank mounting point. The web of the two frames are overlaped about 3" and i then plated the outside of the web with 3/8" plate steel. I weld the plates on and then bolted them with 8 1/2" grade 5 bolts (4 on either side of the splice). The bolts give a little added insurance for that just in case event. As long as you are a competent welder (if your not have someone else do it) then you should have no problem welding frames together. The problem with welding frames is that if it is not done properly then high stress concentrations can form.
I also switched from 1 ton rear spring hangers to 3/4 ton and have had no problems. I bolted the hangers on using 3/8" grade 5 hardware, but i think that this summer I'm going to open up the holes a bit and go with 7/16" grade 8's.
1973 K20 - 350/350/205
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