Hi Scott,
I've got a 79 skylark subframe in my 50, been there for the past 19 yrs. Essentially this is the same as a 79 camaro. Mine rides low, without measuring I would say from the bottom edge of the running boards to the ground is about 6". I cut my subframe just behind the lower control arm rear mount. This is the area of the subframe before it starts to kickdown. The bottom edge of the subframe at the cut point is set even with the bottom edge of the trucks frame rail. I don't have any tire clearance issues, and I run p245 60 15's on 15x7's with 4" backspacing. It is tight, but it's also a big tire for the front. At the front you will not have to notch the radiator support for steering box clearance. My column is a mid seventies montecarlo tilt with the stock steering shaft from the skylark. I retained the factory joint at the top, had to replace the rag joint at the box with a borgeson. I was a little more creative in mounting my subframe than most. I slit the top and bottom of the frame rail just in front of the front cab mount. I then spread the frame rails to the same width as the outside of the subframe and welded it in. Of course I added plate to the inside of the subframe attachment points, reinforced the upper and lower rails where I slit them, and then I boxed the frame from the control arm mounting points back to the crossmember located about midway under the cab. I went this route instead of what you've described because I didn't want it to look like it was subframed. I must have been successful, 19 yrs. later it still drives great, and people ask where I got a frame for my truck instead of just subframing it. I decided to do the work myself after looking at half a dozen trucks done by local shop that specialized in this type of work. Scary. If you've already got the subframe, $1400.00 will rebuild it with a lot of mone left over.
John
