MPO 84 replacing oversized V8 has a higher cold return

I'm replacing an oversized warn out Burnham V8 with an MPO 84.  On the plmbing side most everything seems the same, but the cold return is at the top of the MPO.  All my circulators are slightly lower on the return than the actual return port on the MPO boiler and they are all connected to a 1 1/2" at floor leval where the old V8 returned.   I'm wondering if I need to move all three circulators up above the return port height on the MPO or if I can just take the 1 1/2" return pipe up to the return port on the MPO boiler?

 

On the smoke side, does the flue pipe inside (boiler to chimney) have to be SS or is galvinized OK?  And if SS, which type, 316 or 304?