71 dual field alternator

I have an older 68 single field alternator, I need a dual field alternator to tie into my newer dual resistor/electronic VR harness. I understand 71 model year had the older case alternator but added a 2nd field terminal, then went to the squareback alternator the next year (71 1/2). Does anyone have the procedure to add a 2nd field terminal ($3.00 brush?) to the older round backed alternators? Do you unscrew the ground lug and replace with a brush? Several websites reference the ability to do this but none seem to walk you through it. I dont want to buy a squareback dual field, I already have one, but I just want to convert the older one to dual field (reason: its polished!) I took the brushes out of the dual field and one is straight and one is a 90 degree type, which one to buy for the added one or is it different? Thanks.

Haven't did it either but I'd take out the brush/holder from the non terminal grounded side of the single field alt & eyeball the 2 brush/holder assys from the 2 field & you'll see whether to use the straight one or the 90 deg one then prob adding an insulating washer on either side of the mini screw (or whatever it takes) to keep the screw from grounding the brush to the square mounting nub on the case.