I was able to find a NOS CAT for the '79 Supreme with the 8998598 cat. It was for the equipped 260 V8. I believe the dimensions are the same other than the inlet and outlet pipes are a bit smaller. The gentleman that had that cat was gracious enough to get me the dimensions from that. The 8998692 has the following detents as well 8998782 and 8898876. I would like to confirm the inlet and outlet size of the 8998692 as I am still not 100% sure on that but I can take the measurement of the down pipe that my builder has and measure the exhaust pipe that I have and it should work. It was the length that was the main measurement. The 8998598 length is 22 inches overall. Okay, now that I confused myself, the next tough part is going to be finding a flange that will accept the cat bracket that mounts on the trans bracket. Thanks for the additional info everything helps at this point!Not sure if this will help with trying to find the lenth needed for the cat but p/n for '79 was 8998692. It was used with the L34 (Olds 350), LM1 (SBC 350) & LG4 (SBC 305). Another thing for future reference is the L34 & LM1 used the same complete exhaust from the cat back as with the LG4 with GU2/M20. The LG4 with GH2/GU1 & M29/M33/MV9 used only a different muffler/tailpipe.