Same happened to me, and I totally failed to get them out. I took the head off and had it on the bench. Chisels, dremel, blowtorch, 2lb sledge, nothing would shift it. I ended up taking it to a mechanic as a last resort, something I haven't had to do in years. It took him about an hour with an oxy torch, basically burned them out of there.
Getting the new ones in is easy. They are a nice sliding fit in the ports. Get the old copper gaskets out and clean the mating surfaces for a good seal. If it's scabby in there, the gaskets may not be obvious, but a chisel or big screwdriver will help. They don't always want to come out!
I put new studs and bolts in at the same time, and the Motad downpipes are great. They were shiny for about ten minutes before they began to go a straw colour, and by now they are dark coppery brown. But no rust!