If you're going to chop a rear rail section out for replacement, Leave the K-member in place to maintain the side to side critical dimension while a rail is incomplete. Also, a rear section of a pass rail is much easier to get than a driver side. And they're all the same stuff back there. It's the forward sections that make the difference. Since I don't have a frame rack, I have shot my floor points according to the frame layout plan with a laser level to know if compensation is needed for unlevel concrete. It's not perfect no matter how good your garage floor looks. Mine is off by up to 5/16" from one spot to another. All this matters and you need to compensate for it. But thankfully friends always have tools and skills. I just use various thicknesses of plywood to level floor areas against the highest one. Once I have it mounted level at the trans crossmember and the rear height measurements match, I go ahead and remove the rail section and replace. I use basement screw jacks made into jack stands for the forward section so I can make small height adjustments under the K. You could as easily do it with a couple floor or bottle jacks. You want to have the front draped loose when you measure the rest of the chassis for height, because when you reapply the weight of the vehicle to the front wheels it is going to flex upward. You need to compensate for that in the front height measurements. Don't make them completely equal to the original jig heights as listed on that diagram. Understand that you're also trying to match whatever you have on the other side without making the whole front clip cockeyed. I screw the replacement side up into place 1/8" down from being correct as compensation typically, although I've seen higher mileage cars drape down over 1/2" when the front becomes unloaded by supporting the trans crossmember, while they hang complete. You need to be conscious of the up and down relationship as also applied to wear and tear of what remains to keep it all lined up when finished. It's all geometry and you're changing the hypotenuse with everything you do. Geometry is the simplest form of advanced math. It's not that hard. It's easier than long division. We aren't really dealing with Trig as long as we don't remove the K-member. If the pass side comes down deep, leave it deep and make the driver side low, but not that low because you realize that it's going to be significantly less flexible after your repair than your old original side is. You want them to come up about even, but it is always best if the driver side is just a little lower to offset engine torque. But if you make it as low as the pass side might drape down to when unloaded, it's going to be Way low after putting weight back onto the wheels. Everything has a range of motion, even if you think it's solid. What you're doing is creating the strongest point of a 40 year old metal assembly. Just keep that in mind. All the rest has wear and tear on it and make your best guess based on what your particular car does in loaded on wheels and unloaded to centrally supported measurements.
But if this all sounds too technical or daunting just remember, these things were slapped together on an assembly line the same as all other vehicles of the time, and prior. Made of missed welds and seam sealer, and there isn't really much of anything you could do as an enthusiast who is carefully checking and considering everything, fitting and trimming before welding, that could ever be as bad as what came off that line when somebody was timed for how quickly they could slap fresh stampings into a jig and start welding them up. Your best guess is still going to be better than it ever was before. Because you'll take a minute or a day to think about it and all the time that's elapsed and what that assembly has been through in that intervening time first.
If I can do it, you can. I'm just a guy who wasn't afraid to screw it all up long enough to prove to myself that I hadn't. Then did it again and again.
