Your only limitation will be splitting bores in the stock 250 block. Piston clearance has to be right, and past 40 thou, the thinwall 250 block is marginal. Ford started taking a lot of metal out of the heads and blocks in 1969, with the Cleveland plant making
At 40 thou, with properly clearanced forged pistons, you should be able to go 47% over the stock power level before chemical supercharging. That 47% is based on getting execess fuel to the combustion chamber, without risk. If it was EFI, you could doa lot more, but with carb systems and six n20 nossels and six fuel nossels, you can always get 47%. That's based on the stochometric ratio with a 36% increase in air via the oxgenating effect of nitrous.
The best of the little 200's with a Classic Inline shead are sitting at the 235 to 265 flywheel hp mark. If the 250 is given the right cam to suit its 25% capacity increase, it won't just be a 9% hp increase like it always is with the factory 250 over the 200. You read it right, without an optimized cam and carburattion changee, you only see a 9% power gain over the 200. That's a very different picture to the optimistic 155 and 145 gross hp ratings of the 250 from 1969 tom 1971.a 29% boost over the 120 and 115 hp gross of the 200. Truth is, the rating was a lie, and the SAE net and DIN Net flywheel figures were about 9% different.
If your 250 is optimized to make 295 flywheel hp with a nice big 4-bbl 250, like any decent large lift, long duration Classic Inlines headed engine should, you could add 135 shot and get 430 flywheel hp without risk. The critical loads (writ pin, piston temparature, main bearings) would be less on a nitrous 430 hp than on a turbo 430, or naturally aspirated 430 hp. That's what bottle queens are...something that in its natural state, is significant, but one the bottle, its epic.