The SM420 transmission is a heavy-duty 4-speed truck transmission with a 1:1 4th gear - direct drive. The 49 rear end will have a 4.57:1 ratio. So you are correct that combination is not very freeway friendly. It really depends on what you want to do with the truck. Do you want to pull/tow/haul heavy loads or cruise at 65 under a light load? There have been dozens of threads about T-5 trans swaps into this model/style truck. Search this site and the inliners.org site. But T-5's are not heavy-duty......
For help with the carrier bearing go to the links section of this site and look for Keith Hardy's site chevy.tocmp.com for online Chevy truck shop manuals. Good luck!!
