I'm not 100% sure on this but no, you won't be able to switch barrels with the rotor thats in the 250-3000. If you had a 303 Savage T/D you should be able to swap the 303 Savage with the 30-30, 38-55 and the 25-35 which were all based on the 30-30 but I'd have a gunsmith check the headspacing before I ever took a shot out of one. Chances of finding a 25-35 and a 38-55 T/D barrel would be slim to none and you'd probably pay for more than the original gun was worth. I think I've got another rotor herefor the 303 Savage. Although the rotor maybe wrong anyways, thats the part I'm stumped on. People think they can take a 308 and make it into a 358 by simply having the barrel rebored but they don't realise that the rotor needs to be changed to a 358 rotor, you'd probably be better off trying to have another rotor opened up somehow before trying to find a 358 rotor cause they're about as hard to find as a 25-35 1899. The Savage T/D barrels also have different thread patterns as well, they made 1/4 turn barrels before 1920 and full thread barrels after, not sure if you can switch these around on the receivers. The 22 H.P. is a necked up 25-35 I think, so it might work as well with a 30-30 rotor.
Oh ya, Marbles does sell a new version for the 1899 with windage adjustable knob as well but I like the looks of the old vintage ones over the newer style, the new ones are about the same friggan price as the old ones go for nowadays to.
As for the tang sights, I've got about 5 of them sitting on my desk right now but only one of each maker, Lyman, Marbles, Parker Hale, etc, etc. but if I get a double down the road I'll hook you up, until then keep your eyes out on Ebay. They run at different prices depending on condition.
You've got me thinking now, I gotta dig into this some more. Maybe 44bore might know about the barrels as well.