Heat shield removal with rusty bolts.. How?

The bolt heads are some rusty junk that just crumbles away.. I cant get them off with a pipe wrench either. Last time that I couldnt remove a heatshield I just tore it off because it was so rusty anyway that it was easy to do, but this heatshield isnt rusty so I cant just tare it off.
I need to remove the downpipe from the manifold so I can get the engine out, and the heatshield is in the way so I cant get to the bolts on the mani/downpipe flange. Any tips on removing it? Cutting the shield I consider as my last option but I'd rather avoid it and save the shield.

I didn't know you had to remove the heatshield to get the downpipe off. I know on the A20 the down pipe nuts are accessable from under the car. I guess if you are having that much of a problem then my suggestions would be

1 Since you are pulling the engine, see if you can take the exhaust manifold loose from the head. Then worry about the rest after you get the engine out
2 Take a dremmel to the heads of the nuts, it you can then easy out the remaining part of the bolt with the head out of the car. Only problem is you'd have to pull the radiator and fans to get the extra room you need to work in there

Ha ha, yea sorry youre right, I forgot that the nuts wasnt welded to the flange. :)