If your just using the crossmember then the pickup is too narrow, you have to use the van which is 4" wider. If you using the front frame section then it's best to cut just forward of the front cab mount as this leaves all cab mounting intact. The 78 frame is 1/4" taller than the 55-59 which works out very well since you can have the top of each in line and then leave a 6 or 8 inch "tongue" from the 78 running back under the 5-9 frame from the splice point. This reinforces at exactly the place needed. Steering needs to be worked out as does brackets put on the newer frame for core support/fender mounting. It is hard to lower it much because the tires are under the fender lip. Possible with some deep spaced wheels but would probably have to be some of the new 18" or something similar since moving in a 15 incher soons collides with the tierod end. No guarantee, but I will try to post a pic of a 57 showing the tire position (with a frame clip) at http://s32.photobucket.com/albums/d41/coilover/

Evan