where to find 273 - 340 double lip silicon rear main seal ?

To my knowledge no one makes a double lip Viton 340 rear seal with the wings attached as per the single lip seal . The Mancini and super performance seals come with detached wings that you glue in place in the grooves in the rear main cap . You can make you own using the S0605 seal and by cutting the wings off the S1196 seal to fill the gap on the rear main cap. The factory 340 rear seals are all single lip. The double lip Viton seal was a magnum design improvement and the 5.9 seal is a direct replacement for the la 360 . In the 5.2 they eliminated the grooves and the wings so you have to make your own or use silicone to seal the groove in the cap.