You said that a diesel would be better for mileage but a big block for power. I'm assuming a diesel would be better for towing, correct? (more torque to get going?).
What he said was for power without the changeover troubles go with a big block. He is still advocating that the diesel is the best way to go for mileage and power, but it's going to require a lot more work and money to do than a big block would.
One big question I have is what's your budget on this project? You mention not having a lot of money to do this. In my opinion both routes suggested (big block or diesel) are going to cost quite a bit of money to build a dependable rig. What other parts do you need for the truck? Is it complete minus a drivetrain or is it a shell you're piecing together? Just trying to get an idea of what you're working with.