Hey Andy,
I don't think you need to worry about a 250 hp limit on a built two bolt block. If that were the case then literally 100s of 1000s of Mustangs running around in the US with everything from blowers, turbos, and nitrous are all way over the limit. In many cases 300 to 400+ rear wheel hp over. Occassionaly a stock build will be toasted, but there is usually a such as 20 lbs of boost, too much nitrous, etc. Every 5L Ford I've owned was a two bolt block without failures. The only time I've ever heard anything about a Ford 2 bolt main hp limit is from a Chevy builder, and you can't trust those guys!
As you say, if really worried fit a bottom end girdle. I've got one fitted to my 500hp motor. But the mild 300hp enduro race motor I have has no girdle and it'll be okay. You'll be perfectly fine at higher hp numbers so go ahead and build that thing up a bit!
Ron