I am just in the planning stages of doing a frame off restoration on a 1970 Chevelle. Is it necessary to replace the front upper and lower control arms ?? I am also planning on converting from drum to disk brakes, front and rear. The original 307 was pulled out, and a 400, bored to a 406, was dropped into it, with a TH400 tranny. lots of internal bells and whistles in the engine and tranny. Planning on rebuilding it this winter, due to heavy oil consumption (burning). Is there any advantages to replacing upper and lower control arms ?? and when is it really necessary ?? Thanks in advance.

If there are no cracks, oblong holes, or other issues with the control arms I would just clean/paint,replace the bushings and ball joints and reinstall.
The original arms will work with disc brake spindles.

If they have no cracks, there's no need to replace. If you can blast them, do so - they clean up great. I did POR-15 on mine and then semi-flat black paint and they look good. Just be careful you don't bend the lowers when removing or pressing in new bushings. I learned the hard way about pressing the lower ball joint in more than once....press in the replacement right the first time and all is well. Take it out and do it again and it may not fit snug. My understanding is you only get a couple of replacements. Regardless, I just tacked it in and it's fine. After this rebuild, you should be done with them for your lifetime.

Here's a tip:

If you measure the INSIDE gap between the ears of the control arms where the bushings are pressed in, then go to a local machine shop and have them cut a short piece of angle iron that length; you can use it as a spacer to prevent squeezing the ears together when you (or others) press in the bushings. Did this for the front CA's and rears. Bushings went in easy and not deforming of the arms. Cost - $.50

Good idea. I just took a socket that was the right length and it worked like a charm. I'll also vouch for the El Cheapo balljoint press from Harbor Freight ($30?). That thing was the ticket. I had folks quoting $100 + to do all the bushings and ball joints. I pressed in the whole bunch in less than an hour and get to keep the tool!