My 79 project car is giving me some fits on releasing nuts and bolts that have frozen or corroded in position after 40 plus years in place and a crappy overhaul/engine replacement done by the previous owner. Looking for advice and suggestions to the best means to remove, short of cutting. Should I WD40? breaker bar? impact? heat? any combination?

Go online and order a can of Kroil.  It's the best I've found. Let it soak and reapply. Sometimes heating and applying helps also. It was the only thing I could find that let me remove the four bolts on the rear leaf spring on mine friend's '66 Corvette.  That and a 450 ft-lb impact wrench.  ;)  But didn't break the ears off the diff case.

A good tip for stuck/frozen/rusted fasteners is to tighten them a little first. This will a lot of times help get them to loosen up easier.
GM/AC-Delco Spray rust penetrant #88862628 is the cat's meow in penetrating into tight threads, and convincing rusted/seized fasteners to give up the ghost. Works really well on fasteners anywhere in the exhaust system, and for loosening stuck heat riser valves and such.

 

Many thanks for your suggestions....

PB Blaster has been good to me.  Let it soak in overnight and then spray a little more on when you want to give it a shot.  It'll stink up your garage though.  Joel's suggestion to tighten a little has helped me in the past, too.  

The best $150 I've spent is on this!  There was a few times it took many minutes of "clacking" to bust a nut loose.  Worst one was the steering wheel nut on a C7....go figure?

|UPDATED|5/11/2016 11:29:54 AM (AZT)|/UPDATED|

Air or electric? I have an electric one.....

Mine is also electric.

An alternative to a power torque wrench that's worked for me is a good rust penetrant and patience. Apply the penetrant, from both sides if possible, let it soak, maybe add more -- 2-3 times, even. Soak that baby with penetrant. Turn the nut or bolt to loosen it but avoid excessive force that can break the fastener. Add some penetrant. Turn it in the opposite direction. Apply some more penetrant. Repeat. Always baby steps, and adequate but minimal force. What you want to do here is open up some paths for the penetrant to... penetrate, deeper and deeper in between the internal and external threads. Don't be in a hurry. Continue repeating loosen/penetrant/tighten/penetrant until the nut comes off or the bolt comes out. It you don't want to take the time to do it slowly, think about how long it will take you to recover -- not to mention the aggravation -- if you hurry and twist the head off the bolt or the end off the stud. If a nut's readily accessible but so rusted a six-point socket won't turn it, a Dremel tool with a thin cutting disk can be used to cut off the nut, being careful to minimize damage to the mating threads.

I also like heat till cherry red then quenching with PB blaster. It cools it down somewhat and lets the penitent to creep into the rusted areas. And yes always try to tighten at fist. Tapping with a hammer also will let penitent flow where you need it. Remember when it starts to loosen to spray it and re-tighten a bit as someone mentioned slow and easy does it. Make sure to also swear a lot............it don't really work but you need to do it anyhow......it's a "guy" thing!