Removing the exhaust headers from a stock exhaust

Hi all! I was wondering if anyone could give me a bit of advice. I'm trying to refit the stock exhaust for my 2005 sv650s k5: the original was hacked to pieces by the dealer that fitted an aftermarket end can to it, so I've found a replacement full system. It looks in good nick but I can't get the front header out of it so that I can get both manifolds back into the bike.
The rear headers on both the original and the replacement exhaust move around freely, but the front headers on both of them seem seized in place. Am I doing something wrong? Does anyone have any tips for loosening them up?
Thanks in advance.

If you can get a bit of heat into them it can help free seized metal. A blow torch for a few seconds on the joints should sort it. Failing that, a good squirt of trusty WD40 is the only other way that I can think of.

Simple: Don't take the front header off. You can fit the system without removing it, and that's the way it's supposed to be done. As long as the read header stub is in the rear cylinder, you push on the rear header onto the stub first and then slide the front one into place on the engine with a bit of fiddling.

Thanks guys! I got it on in the end by removing the oil cooler and then just fiddling it in. Took ages to get the rear header onto the stub though: the wire gasket was knackered and kept catching. Suppose I'll have to replace that when I've got a free afternoon.

Next question: I've just fitted a new radiator to the bike (crash damage ) and it doesn't seem to get hot when the bike runs. Even when the engine is up to 80 degrees, the pipe into the rad is hot on the right (starboard?) side but still cold on the left. I thought there might be a blockage in there, but squeezing the pipe on the left with the filler cap off does cause the coolant level to rise. I can't seem to get any air bubbles out of it either. Any tips?