I am considering (i.e. in the process) of doing this conversion. My reasons are: cost and availability - $400 for a Brembo front set of lowers is a bit high and left lowers are easy to find; better brakes - I can live with decent (still, not great) ATE dual brakes; it's period correct - not that the bike has any collector value, but it looks right; it is not permanent - the original right lower can be reinstalled. Drilling and reaming a new hole is no problem and cutting off the tang to weld on the other side may just be the excuse I need to get a spool gun. I think the tang has no function - other than looks.
I have another brake line and rotor with pads, so another ATE caliper, a piece of M10x1 steel brake line, and a brass brake line tee are needed. I got a set of /7 lower forks for $80.00 shipped and can live with losing that amount if this doesn't work out. I'll probably swap the ATE caliper to the right side to test the modified fork and rotor operation before finishing the conversion.