Have you had a chance to weigh your 73 duster yet? I ask is because i am also building a 73 duster and would like to see what yours weighs at. If you are going to race it with some slicks i would skip the 8-1/4 and go to a 8-3/4. here is what i did to my Richmond 4.10 geared 8-3/4 on my 5th pass at the track. this was with a 3520 pound big block dart and M/T 275/60/15 ET street radial. after i got home i installed a used set of 4.30 gears left over from my 65 Plymouth. so far the rear is holding up but i know i am on borrowed time. I have a Dana i am going to install some day when i get the money. well the good thing is if you do brake your 8-1/4 at least you wont have to pull the trans to check the sprag like i did.lol also i would like to see some pics of your duster. thanks mopar65