I was trying to replace my heat riser and needed to loosen up the front crossover pipe.

Well all the nuts turned on the LH and RH side, but on the RH side there are 3 long bolts that run through the manifold. The nuts will come off but the bolts that are 3 to 4" long are seized in the channel they are in.

I sprayed them with some blaster. I will try again but any other suggestions? The pipes are fine but I need to loosen this up to put in a new heat riser.

Do I heat the bolts next?

 

Tap with a rubber mallet or brass hammer, be gentle and patient

 

Not sure what it is you call ‘blaster’ over there but I completely stripped my 59/98 down a few years ago for painting and any stubborn nut or bolts I hit it with ‘freeze n release’ then CRC penetrant and alternated this for a while.
The freeze n release because it is cold contracts slightly the rust and stuff holding the bolt tight which the movement causes a hairline crack allowing he CRC penetrant to get into that crack.

I used this method on a 54 Chev truck heat riser also over a few days and wrapping the heat riser in a penetrant soaked rag overnight. As Toymobile said tap away and all is still working to this day.