looking to swap LSX in 78 k20

so its been a while since i been on here. have a few questions tho. im looking at swapping out an lsx motor into a 78 k20 scottsdale. it currently has sbc 350, transmission is the SM465(3 speed w/granny) with 4.11 gears. i havent decided yet which ls motor i plan on going with whether be 5.3, 6.0, ls1, etc. im looking to make the truck more reliable and effecient. i am wanting to keep the same transmission and possibly change the gears, its still an offroad vehcile and a work vehicle but i wanna be able to daily drive it. i will take suggestions on what some of yall would do.
what is needed to adapt to the SM465 and what gearing would you suggest to put in to do what i want. i know im not real specific. this will be my first swap and i am planning on doing all the work myself. thanks in advance for any input