Test-fit the new framerail and clamp it in place. Hopkins suggests leaving a slight gap between the original frame and the repair that will ensure sufficient joint strength. With the sleeve, this will easily become the strongest portion of the car's rear subframe. Duplicate your measurement technique to ensure the new framerail is properly positioned. This may also include the rear shackle mount for the leaf spring. It might even help to make sure the bumper will bolt up. If you are within 11/48 inch of the original factory specs, this is probably better than the factory performed back then. After tack-welding the repair in place, you can perform the plug-welding and then measure again to ensure nothing moved. With everything square, the entire repair can be welded, with attention paid to not putting too much heat into any one area. If aesthetics are important, you can grind the welds down slightly, but not enough to sacrifice weld integrity.
Finally, we'd suggest blasting and painting this entire area to prevent further corrosion. If all this is too much work for you, we're sure there are body shops in your area that could handle this effort, especially if you located the donor framerail. But don't be intimidated by the fab work. Consider this: For the amount of money you would spend to have a body shop do the work, you could afford to purchase a 110-volt MIG welder from Sears/ Craftsman, Lincoln, Miller, or any other reputable welding company and own the welder once you're done with the project. We did that several years ago with a trunk floor repair with very little prior MIG-welding experience. Who knows, you may actually enjoy the experience of saving your car with your own two hands.
It's a Dog
Eliot Stepka, via CarCraft.com: I am 17 years old and seeking advice. It has taken me two-and-a-half years working on and working to pay for my '70 El Camino to get it to this point. It runs and drives now. I am having some problems tuning my motor, a '99 5.3L Gen III. I converted it to a carburetor using the intake from GM Performance Parts for the LS motors along with the standalone ignition system from MSD. I am also using a Holley 750-cfm vacuum-secondary (PN 80508S) with 70 primary jets with a 65 power valve. In the secondaries, it has a fixed metering plate with no power valve and a 25 squirter on the primary side. I have tried tuning the carb and changing the timing chips, but nothing seems to help. It sounds great when it's not under load, but when I drive it and nail the throttle, it falls on its face. I've also discovered the secondaries are not opening, and I know the vacuum diaphragm is OK because I just changed it and tried a softer spring and it does not seem to help. It has 20 inches of manifold vacuum at idle. Can you think of a reason why the secondaries aren't opening?
My drivetrain is a race-built TH350 with a custom-built adapter converter by Cone with a 2,000-stall. The rear gear is a 2.73, so I know it's not going to be perfect, but it should still light them up. It also has custom-built headers from Street and Performance in Arkansas. One of my concerns is the vacuum system. I have a T running off the back of the carb to a PCV valve on the driver-side valve cover and the other line running under the intake to an elbow running off the other valve cover. There isn't much room for a breather because of the coil packs on the valve covers.
Jeff Smith: You mention the engine is a "dog" and it's slow on acceleration. The Holley carb you mention is a 750-cfm, 4160-style Holley. The specs on this carb use a 72 primary jet with a 65 power valve, while the secondary uses a metering plate with a drilled secondary "jet" orifice that is equivalent to a 75 jet with no provision for a secondary power valve. The accelerator pump squirter is 0.025 inch with a plain (no color) secondary diaphragm spring. Since you are using this carb on a rather large-plenum single-plane intake manifold, it's possible you'll need a bit more jet in the primary. You mentioned a 70 jet but stock is a 72. We'd suggest upping that to a 74 primary jet to compensate for the reduced signal resulting from the big-plenum intake manifold and the rather slow-responding, straight-leg boosters. If the engine had a dual-plane intake, this additional jetting might not be necessary. We'd also try adding a slightly larger 30cc accelerator pump squirter and see if that helps.