I also run a 505ci in a 70 Challenger with a 3:55 gear. Here is my 727 modifications:

Stock front drum with 5 thin Alto Red clutches and Colene steels
Modified rear drum with roller bearing 4 Alto Red and Colene steels
Billet rear support with roller bearing
A&A super sprague
Billet front and rear servos
Billet band struts
Turbo Action manual valve body forward pattern LBA
Aluminum deep pan
PTC 9.5" converter 3000 stall

No problems in 11 years and I built the trans myself with the help of the late Carl Monroe's book