Earlier this year, I put a new headpipe on a very original 89 245. Original headpipe broke, original cat was saved. What a chore. The bolts in the cat flange were no more than nubs, and rusted solid in place. They're threaded into the converter flange. Getting them out in one piece is unlikely.
The chances of an original 1986 part coming apart correctly are small. I spent about 2 extra hours dealing with grinding and drilling out the three bolts, using my tungsten drill bits. Not worth the extra work unless you really must save the $120 that a new cat costs. If you're paying someone's shop rate for this work, it will cost as much or more to deal with this if even one bolt breaks.
Add that to the fact that an original converter is way past its lifespan anyway, and replacement makes sense.
If the car's a keeper, put the new cat in at the same time as the headpipe, and you'll never have to worry about it again.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 230K, 88 744GLE- 220K, 82 245T-181K Also responsible for the care and feeding of: 88 745GLE, 231K, 87 244DL, 239K, 94 855GLT 189K

If the cat is not giving you any problems, why waste $100? The dimples are OK. Is your muffler guy trying to sell you something? Is your header pipe leaking or is it just cracked? If it is leaking, it will need to be replaced. If it is just cracked you could always wait until it leaks - 50 60K down the road.

Klaus
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95 854T, 88 780, 88 245