Yes the 1/2 shoulder is plenty for you mount. Don't forget Rocksett.
M1B ultimak will fit, however, if you wanna bleed off more gas suppressed(less rearward emission), go with the M2B ported. Just my preference.
Adjustable piston will fit, but there is some measuring and adjustments you have to make on your end.
(I've rebarreled ak74s into 556s) before the market caught up)
Side not: maybe think about using a RS Regulate side mount instead of an ultimak. Since you'll be using a fixed stock, optic preference is up to you. I have some ultimaks I like, but adding side mount is nice for other optics.