I have a 70 340 Duster with a 727 and an 8 1/4 with what I believe to be 3:91 gears - but not Sure-Grip. ( 4,000 rpm at 60-65 mph) Will smoke the RR tire easily. I would like to switch to a SG with 3:23's First of all - Why build a 3:91 without SG and secondly what is a good donor vehicle? Also how can you determine it's a SG when it's in weeds and mud. I'm on the East Coast of Canada and if the rust hasn't killed it the Environmental laws have had it crushed. I'm fairly new at this so appreciate any info. Thanks.

pre-96 8 1/4s will be the correct 27 spline for the old rears. if you can't turn the wheels in the mud, but can take the cover off..do that and see if you can see some springs in the middle of the unit.

Dakotas, Cherokees, some small duty Rams will have the 8 1/4 in them. old Diplomats and Gran Furys, Fifth Ave's as well.

the key is the pre 96. newer will have 29 splines.

Once you get that, you'll need the pre-76 Race/carrier bearings which press on each end of the unit. I believe the newer ones are larger.