I picked up My rear engine dragster a few years ago. The guy had a shortie 727 and a 493 RB with Indy Heads...He was currently going 5.40-5.30's in the 8th mile 60ft was in the mid to upped 1 teens or so.
I knew a 3 speed wasnt going to cut it in the dragster...It was a very heavy for dragstr with driver well over 2000+ lbs. But the driver is big guy too 6'4 and 270 lbs.
Anyways
I purchased the Power Glide Hand Book By Carl Monroe (sp)?
Anyways. I went through about 6 cases before finding a good one. Purchased a shortie stock 1.76 gear set. Add a Billet clutch hub, new servo, 300M 350 input shaft picked up some thin clutches and plates to fit as many clutch discs possible in the front IIRC it was 7, and ended up with 5 out in the rear. Added a deep pan and trans brake. Set pack clearance to about .100-.120 in the rear, and about .070-.090 in the front, bought a used Coan converter, thow'd this junk together..First time I built a Glide. Went out and ran way down in the 4's in the 8th mile, and pulled consistent ONE flat 60 ft....This wasnt no one pass wonder either, dead heat in July...
Two seasons later, the trans is still working fine with nothing more than stock parts.

For the guys who probably picked up E/T in moderately heavy cars, with moderate HP over a 727..its simple...the cars wasnt set up properly, and was spinning..something...