how much would i gain from swapping out the 177 on my 525sc to a B&M 250? is it worth the hassle or better off to jump to a 671?

go bigger at least a 6 . i have had and seen a lot of problems nwith the b&m 250

Save $$$$$$$$$$$$$ for gas...

What carbs do you need to run with a 671?

the 177 is maxed out already. Spinning it harder makes a lot more heat and a little more boost.

the 250 will give you additional "horsepower headroom".

The 6-71 or 420 MegaBlower will be more than enough blower for anything you can sensibly do to a 454.