1971 Chevy C-10

[Lichen]

Greetings
I have a 1971 Chevy C-10 short bed fleetside 250 ci 6 cylinder, wood bed. I bought the truck in 1983 with [77,000] miles,now it has [465,761 miles] [original engine/no rebuild] . It still runs great on the flat [freeway] but has no power to climb hills. In 1984 I changed it to dual exhaust, from 3 on the column to 4 on the floor with a 3/4 ton transmission [compound low] and in 1998 I put in a positraction rearend. Because the rearend was 6 bolt, I changed the front drums also, over to 6 bolt, allowing me to run it with
9.50 x 16.5 tires.
This has been one great truck I have driven it all over the western United States,Canada and Mexico. With allways enough power to get me anywhere I wanted, plus I used it for my hauling business and fully loaded it performed any job demanded of it. I also drove it for years on dirt roads [and what could be hardly called a road] off the grid, in wilderness areas, like the Baja in Mexico and thru mountains and deserts under extreme weather conditions.
Because of the headers, it allways sounded like it had a 350 in it and over the years, everyone was all ways amazed that it only had a 250. I allways imagined that it would be with me until the day I died. Because of the rising gas prices [San Francisco 3 weeks ago $3.79 a gallon] and the negative impact, fossil fuels have on our environment I bought a diesel truck [1985 Ford f250 extended cab utility body 6.9 Intenational Harvester engine IDI] which I run on homebrew biodiesel and WVO [waste vegetable oil].So instead of it costing me $300 or more a month to get around it costs me no more than $10.
I am a backyard/street mechanic and I am also proficient in auto body repair, custom paint and fabrication.I am going to restore my truck for sale and I am looking for a word up [the engine also has a rear seal leak and I have to add a quart of oil a week]. I could either rebuild the 250 engine [better mpg] or upgrade to a 350 which it seems, that is the engine, that most people seem to prefer,what do you all think?
Not to abandon my life long admiration/love for Chevy/GMC, [before purchasing the C-10] I drove a 1952 GMC Suburban for 10 years [stolen, never recovered].For the past year [with 2 of my friends] we have been restoring a 1974 El Camino SS where we swapped out the 350 gas engine for a 1986 6.2 truck diesel, turbo charged [Banks sidewinder], propane injected to race on biodiesel on our local tracks.The 6.2 is a perfect fit but because it's even heavier than a 454 we beefed up the front suspension and braking system installing all racing parts.We will have a website up in another month or 2 and you can check out our ride "Grease Runner" at www.alternativefuelstechnology.org. Peace Have a Great Day Motor On