Thanks for the suggestions, guys.
Luther, I'm not yet a welder, but even if I were, it seems to me that the two mufflers have very different configurations. If the Noise Suppression version employs an honest-to-goodness 1" x 1-1/4" (straight) nipple, then installing it properly would mean removing the current 90 degree nipple from the block and inserting a nipple that could be found or fabricated at any plumbing supply outlet. Then the only issue is finding a "Heat Baffle" to fit. Again, I have no idea what the Noise Suppression option looks like installed. If I cut the elbow off the Noise Suppression muffler, as you suggest, I would have to be sure the diameter of the "stub" mated up to the existing 90 from the block --but again, it seems to me that the angle of entry into the two mufflers is different. Of course, this is no issue to someone who can weld.
Reposting what got eaten by the archive bug:
As I mentioned earlier, the "Heat Baffle" for the OEM-style muffler, P/N KH-237679 on Tractors w/serial no. 438962 & above is depicted on Parts Lookup for the K321A:
(I think the holes, from bottom to top, are the stud #7 in the tightening sequence, bolt #8, bolt #1, and stud #4 with the hole for the muffler screw at the top of the image.)
The Heat Baffle for an engine with the Noise Suppression muffler is listed under the same item#, but Parts Lookup tells me that it is No Longer Available (NLA)
I guess what I'm looking for is a picture of a tractor that came with the Noise Suppression muffler and that still has the original heat baffle on it, as well as the exhaust hipple.
Once I know what I'm looking for, I can go shopping (or fabricating).
A final thought: Of course, I'm going to all this trouble to have a quieter tractor; but can anyone tell me if the Noise Suppression muffler is really quieter than the OEM version?