The 6-71 is a big boat anchor (I've read 65 lbs. without mag end plates, needs big brackets, high inertia to spin up) for your purpose, and although capacity is high running it slowly means high leakage at low RPM.
For a roots type (boost at low speed) a Weiand or B&M small GMC replica is a better choice if you can find one used ($$$ new).
The M90 is a bargain, but remember that no OEM Eaton accepts a carburetor - no flange, intake in the wrong place. IMHO an M90 could be converted to top-fill with some surgery, and rotated 90° to use side-draft carbs and keep the hood clearance.
All the centrifugals (Paxton, ATI) must be spun faster, and produce less boost at lower speeds - which may help if your static CR is too high?
