Long days at 150 degrees will oxidize the oil, turning it into varnish. This is bad. Getting it good and hot will soften the varnish so you can get it off. This is good.
If you take Motorman's advise and get it up to 350 degrees, just don't forget and leave it that way for a week, and you should be fine. I like Brett's suggestion much better -- if I were going to do it in my shop with heat, I'd use a Monocoat gun.
Personally, I use whatever straight alcohol I have at hand -- usually isopropyl or denatured. But I'm careful to remember that I've used it, and that it attracts water like a bandit. I'm of the opinion that if it's that hard to loosen it's time to take it apart and crock-pot the pieces.
Whatever else you do, make sure you're getting solvent (alcohol, fuel, or Brett's light oil) into the upper cylinder, either through the exhaust ports or down the glow plug hole.
Consider crock-potting the thing if it's really that bad.