Compression check

You have a previous post about selling your car. I'd not use a head gasket sealing product to sell it, or if I were keeping it. I recommend you change at least the one head gasket, or sell it as is.

Sealers are only a temporary fix until you can do it right no matter what you may hear, so I would fix it because you won't get s@@t out of it otherwise.

+1 on above. Never use any sealing products in a motor. They don't really work and cause a lot of "collateral" damage inside the motor, radiator and heater core.

Check out car-part.com for a searchable list of salvage yards in your area. Look for a SOHC 4.6 from a 2003+ Grand Marquis, Town Car, or Crown Vic. This will give you an idea of what a whole salvage motor would cost to give your Mustang a new heart.

Knowing what I know now if I needed to do a head job on a Mustang likely I would do the work on the engine stand (not in the car). The motor is really not that hard to remove. I have done it in my drive way using just hand tools and an engine crane out the top. It's sooooo much easier doing major motor work on an engine stand.