Hi, I have a 78z that somehow has 0 rust anywhere except the rear foot of the right frame rail. The bottom of the frame rail is rusted out down there for a good 8 inches. The other frame rail is perfect, with no rust. Floor pans are new. No rust on the suspension or body. So don't really want to scrap it ?. Not sure how it rusted in only that one spot also. But onto my question. I should mention that I am new to dealing with rust, so am seeking guidance here.

Would it be better to buy 2 new frame rails and weld the both on or do you think I can just cut that section in the back down to good metal and rebuild those last 8 inches of it? I can weld pretty well but I do not have a rotisserie or anything like that. It would just be a pain to have to do both frame rails and never is as easy as just welding them on top of the old ones. I wouldn't be so apprehensive to weld new ones on if that wasn't the only spot of rust on the car.

I would cut out the rust in that section and weld in a patch but it is hard to believe those eight inches are the only rust.  Anyway open it up and have a look down the rail and go from there.

Yea, I agree.  Why would you want to replace the good one, anyhow?  If you can get away with a patch like that, do it.  See what you have to work with when you cut it open.  Is it under the battery tray?  That's why it rusted out in that spot.

29 minutes ago, 26th-Z said:

Yea, I agree.  Why would you want to replace the good one, anyhow?  If you can get away with a patch like that, do it.  See what you have to work with when you cut it open.  Is it under the battery tray?  That's why it rusted out in that spot.

It is in the rear of the car no where near the battery. The other frame rail is 100% perfect, undercarriage has minor surface rust, body is 99% good. But the rust in that corner is real bad. Not quite sure how it only rusted there. nothing back there at all lol. Do I need to support the car in some way while I cut the frame rail down? 

Post up some pictures to make it clearer where you are working

1 hour ago, Pochie45566 said:

Do I need to support the car in some way while I cut the frame rail down? 

Well you may have to put the car up on jack stands to get enough room to work but the area around where you are cutting won't need any support. Looking forward to the pics.

Here ya go. The reason the hole is so large is because I cleaned all the flakey metal out. 8-10. Top of frame is fine 

Wow, maybe some water got in there and couldn't get out until it rotted, the top of the rail is actually the floor pan.  Seeing that it is the floor pan rail that is rusted I would replace the whole rail.  I would also do some exploritory surgery on the other rail just to make sure.

The floor pans look great in my opinion. Look new. That’s odd. Well I’ll order some mad dog frame rails and weld those in here soon. 

1 hour ago, Pochie45566 said:

The floor pans look great in my opinion. Look new. That’s odd. Well I’ll order some mad dog frame rails and weld those in here soon. 

Be sure and make note of the roof joint thread I started, and follow John's advise on high to install the rails properly. More weld is not always better!