LS 500 hp would go with bigger size will be easier less compression can run on pump gas. Don’t discount carb or throttle body injection factory computer can be flashed but need someone with dyno and knowledge of system.
Stock 5.3 300hp easy 400 hp
6.0 or 6.2 500 would be nice.
Floors first, rockers second and cab corners last. You will need to weld in temporary braces so when you cut cab and rockers out the rest will not distort.
Make sure you have metal to weld to has cab been blasted to bear metal?
Almost all sheet metal is built in same plant in China boxes are all marked and even though prices are different almost
always identical. Sherman and AMD are ok just keep in mind new metal is not exact to what GM built truck with you will most likely need to fit all panels.
Lifted? I run Bilstein cost more last almost forever if it is lifted may still have something for your truck. Summit sells many brands including Bilstein that’s where I picked mine up 1 ton 2wd.
You definitely have a siphon or drain back issue if it sits for a month or two fuel will evaporate. Fuel pumps can be an issue make sure it is not leaking into crankcase.
6.2 is not a bad engine would use it if already installed military had plenty they are a workhorse not racehorse Cummings swap would get you there.
Cummings would get you 20+mpg never do that with gas.
Aluminum rocker arms can fail more than steel or chromoly I would get just one and check the pushrod I think some valve lash issue could've caused your broken rocker arm
Summit racing has a carter fuel pump designed to be run with a mechanical fuel pump also great price you will have to run a regulator before the carb pop will last forever if you follow installation instructions
Big tires are a killer dropping down tire size will also decrease the rolling wait that will help with your mileage anything bigger than 33 just gonna hurt you 38 you're just tough to get better than 10