Besides the obvious, cutting and balancing the driveshaft, new yoke, what other modifications am I in for? I have a 67 Elky 396, and am going to switch since I killed my TH350. In the archives, it said that 67 Chevelles have extra holes in the frame because the TH400 was offered as an option, are the same holes in El Caminos?

I would guess that the ElCamino has the same extra frame holes for the crossmember locations. One other difference is that the th400 kickdown is via a switch vs a kickdown rod. You will need to moount a switch on the gas pedal or carb. One wire to ign and the other to the tranny connector on the drivers side. The switch should close when pedal is to the metal.

If the holes aren't there, then put them there. I've got a whole assortment of non-factory crosmember bolt holes on my frame. The extra unused holes are "speed holes" (according to Homer Simpson), that make the car lighter/faster!

B&M sells a kit that mounts the required switch directly on the TH350 kickdown bracket.
www.bmracing.com (http://www.bmracing.com)
My '69 El Camino has extra crossmember holes from the factory and I would assume that they all did since the TH400 was also available in the El Camino.