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.