You should have a 3.23 gear.My 69 Bird runs the same,3000 RPM @70 MPH,with 3.23s,I have 235/60/15 on the rear with a 4 speed Saginaw,not sure on the tranny gear ratios though.The best way to tell if you have a LSD,is to take off the cover.My car will spin both wheels if they get equal power,but I DO NOT have a LSD,just an old fashion open diff.