Converting a 671 GM core into an nice piece of equipment, involves quite a bit of work.  A complete core conversion includes:

Aftermarket, heavy duty front and rear bearing plates

New blower bearing and seal kit

Machine blower case 100%

Machine and prep rotor set

Add two additional taper pins in the front rotor shafts

Set rotor timing, front, rear and bottom clearances according to application

Provide new hardware and grade 8 blower bolt kit

The average cost for a GM 671 core conversion is: