81 Spider Turbo
I want to remove the fuel tank to flush out, but the bolts wont budge. It is to the point where I think they will snap off if I apply anymore torque. I have applied plenty of penetrant without results. I have used PB Blaster. Any good suggestions? If I do break them, how hard is it to replace the bolts? Thanks.

Try tightening them just a tad. Tap the top of the bolt to see if it might help break the corrosion a bit. Both helped with corroded bleeder bolts on the brake calipers on my cars.

those are studs and not bolts. you have access from the bottom of the car, so they can be replaced with bolts.

ya if they snap just drill then out and replace them with bolts
use stainless steel as they wont rust