I know that this is an old post but I just ordered a replacement oil pan for my L83 swap today and the original GM part number you gave for the Corvette has been changed. The new number is: 12679949 and it ain’t cheap!!!!

For you, OP and anyone else interested in these adapter plates make sure you are using the correct plates. Turns out many were having trouble including myself, alignment was off by about 1/2”. These plates were revised at some point but even last month i go a set of the original style, see pic attached, the plate is sitting on my car. The newer version has the 3 mounting holes dropped down about a half inch or so and it fits perfectly. See attached picture mounted on my car, you can see the bolt holes lowered and the bottom of the plate even extended. I was Also using stock mounts with the stock frame stands at the time of instal. I just now got the energy suspension mount (was shipped two different mounts somehow and I’m waiting on the other correct ES mount to show up) and haven’t test fit it yet but there should be no difference as these mounts have the same measurements. Also my DS plate is cut as advertised to fit the mechanical vacuum pump, I’m in the middle hole too on my 68. Have to notch the frame a little but it all worked out. ES mounts also require slightly longer hardware due to the extra plate.