Couple points.
What do you want to end up with? If the crustang username is correct and you want to end up with a nice daily driver, bodywork will cost you a ton of money unless you plan to undertake it yourself. And just know it's not as easy as the youtube vids would have you believe. That's why I'm spending $40K to have my wife's MGA (cherished car, been in the family since 1973) bodywork fixed and painted, while I undertake the mechanical resto mod. I KNOW I can't give the MGA the respect the bodywork deserves, so it gets farmed out
OK, let's say for the sake of argument you want to keep the car. You want to put a 30 year old engine in it. A truck engine. And we all know how six cylinder truck engines were the darlings of fleets being driven by employees who don't give a rat's... er, fanny. So your $700 engine becomes a $2500-3000 engine by the time you knock the bores out to the next oversize (if they haven't already been bore to max oversize) and refresh the valves. Now you get to do suspension and/or changes required for the additional weight, height and probably width of the engine.
Does anyone make a swap kit? Are you a decent fabricator? While I may be interested in a 300 inch Mustang, if the swap looked like it was done with an axe and a hammer, I'd walk away. And it's not just about selling. You want respect at any car show/cruise? most people know a hatchet job when they see one. If you are a decent fabricator, delete this paragraph.
I kinda reject the implied argument that 300" parts are significantly cheaper or more available than 250 parts. True there is a larger supply base for the 300" engine (as it's attractive because of the removable intake). And you may need to wait a day or two for a 35 year old part from a supply house. And Vintage Inlines may not have parts in inventory as he's a small change (no insult intended to Matt, but he's no Speedway, Jeg's or Summit) operation intending to supply a single engine architecture. He's been focusing in the last coupla years on getting the alloy head back in production. But going to the big guys I think you'll find that parts that are available aren't significantly more expensive that 300" parts
You're not going to get a bargain basement 2V or aluminum head. More demand than supply. What you may want to do is read the tech archive and see what people are doing with the log heads. Including cutting off part of the log and building a custom intake system. What people who run this engine are hearkens back to the early days of hot rodding, where guys sat alone in their garage cuttin' and grindin' on factory parts since nothing was available. No corner speed shop (remember them?), no big manufacturers churning out speed parts. Just guys with an idea.
And I'll leave you with an example of what I'm talking about. A guy in his garage with an idea to improve carburetion on his 200" six in an MGB