A military 302 can be converted to fit in your truck, but its not a easy conversion in my opinion. A civilian 302 is almost a direct sway depending on what year range you go with. A 270 or 302 would be a great upgrade to your truck, along with the 4:10 rear. The 4:10 rear change will be a quicker upgrade than changing engines. I would start there and then see how you like the truck and then if you need some more power then start looking for a good 270 or 302 to drop in.
On a another note....
"248, etc., pans will not fit 270's, etc., because of the longer trove."
I used my 228 oil pan on a 270 swap back in 2008, on my previous 52 GMC half ton without any issues. My 45 GMC has a civilian 302 in it right now with a 228 oil pan, again no issues. Not sure where your info is coming from but I've made it work without any alterations.
Yes, there are different oil pans between the 228, 248, 270, and 302 engines. but from I understand the differences have to do with oil capacity, not a "fitment" issue between the blocks.
1927 Chevy Capital 1 Ton truck - basket case
1941 Chevy Suburban - Parts only
1949 GMC Suburban - 10 year project
1945 GMC half ton truck - Driver
1946 Chevy COE - Might restore one day...https://photos.google.com/u/1/albums