There is tons of aftermarket sheet metal in the catelogs.
Metal aint the issue, money and time are.
The head gasket might be the issue, a cracked liner will also pump coolant. Wont know until you scope it or unbolt it.
If you figure on the "worse case" scenerio, you wonxt be surprised.
And here is how I negotiate it...
He says "head gasket"..I would reply, "or cylinder liner".
Spare tranny in bed..."is that the original or a replacement?" Was it rebuilt? By who?
Any "questionable" aspects of the vehicle work in your favor to chip at the price.
If you think it's a $***.XX truck, then once you have layed out all the "negatives", reach in your pocket and pull out your off and tell him, this is it. And let him be the next person to speak. Under no circumstances do you say 1 word until he does. If he hesitates and him-haws, pick up you offer and thank him for his time. Do not hesitate and Walk away. Don't look at the vehicle.
The key is "zero" emotional attachment.
You know not to grocery shop when you're hungry.
Don't car shop with emotion. It's a head game.
The other thought might be it's a doner for another truck. Two doners might make 1 dependable driver.
Just some thoughts.
Good luck.