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If there is no flange just in front of the cat then it has to be unbolted from the manifold, If this is not possible you will have to cut pipes and purchase sleeves to connect afterwards hope this helps.
Sounds like the catalytic converter. Remove the cat and put in a test pipe or just run it with out anything "it will be loud" and see if it drives normal. If it does replace the cat. If not check all plugs, wires, dist cap and rotor, and coil. I wouldn't hurt to clean the throttle body as well.
The upstream O2 sensors will be before the catalytic converter just down from where the exhaust pipe(s) attache to the exhaust manifold. The downstream O2 will be right after the catalytic converter. Once you crawl under the vehicle they will be pretty obvious where they are in the exhaust pipe.
It's not worth doing all that conversion, buy the time you spend the time and money to convert the 89 4.3 to work on the 98 blazer, it would be cheaper to install a 98 vortec rebuild. The 89 4.3 have a TBI fuel injection system and the ecm on the 98 will not work and the bad news is that the ecm on the 98 also controls the transmission shift points and the 89 will not, so even if you swap the ecm to have the 89 4.3 engine work. the transmission will not shift.
Sorry for the bad news but it would be cheaper to replace the 98 engine with the years of 1996 to 2001. Good luck and hope this helps.