Rear end info -- 68 Shortbed

[JJchamp48]

This is hat I've got:
-- 68 shortbed stepside Chevy 1/2 ton.
-- HD rear coil springs.
-- Factory 4:11 rear end. (for some reason I thought it was Posi but am not positive. factory option #4H04CA). I have not gotten # of diff yet.
-- Powerglide tranny was factory. Someone swapped it out w/ a 3 spd, but I have orginial tranny.
-- was 6 cyl, po installed 350 which blew up. I am installing a 65 Vette 327.
-- tires are 275/60/16, 6 lug

What I want to do:
-- I live in the mountains but don't climb them regularly
-- Being rural, I don't drive in the city much, just on trips to town.
-- most of my driving is 55 or better
-- I will be travelling in truck to races, kid's college, visit family, etc which will be mostly interstate speeds.
-- I want a cruising gear more appropriate for my use

This will not be a daily driver. I have a GMC for any heavy hauling, firewood, etc I need to do.
I was thinking about 3:42 but was told I needed a smaller carrier to switch to that ratio.
I am also thinking swapping rear ends would be easier, but not sure which ones interchange.
Any help would be greatly appreciated.