Exhaust Downpipe Rusted Studs/Nuts

Hi All -- My 1987 B2000 has an exhaust leak -- After the exhaust manifold there's a short section that goes down, I think there's a cat in there, the first exhaust pipe bolts up to that upper short section. The upper part has three studs, then three nuts that hold it together with a gasket. The studs are rusted and I can't get the nuts tight enough to seal properly. In fact now they are hard to get off.

If I could get in there with a die I could chase the threads and it would seal but there isn't enough room for a die. Any ideas short of taking the whole thing off? Is there such thing as a small die that's the size (diameter) of a long socket? I can't be without the truck right now so trying to avoid the whole manifold removal ordeal. Thanks for your input....

weld it

Wow, tough situation.

if all else fails,I have a 2200 exhaust manifold with downpipe and cat attached.all the way down past the air injection pipes.If u want to replace yours,mine is available..BJ