Welcome to the 'bolt F100!
That 450 is going to be a heavy truck, so with a 350 you're asking a motor that likes to make its power turning fast to work hard going slow. And the T350 is not a heavy duty transmission. I think if you want to use this truck as a truck, this is the wrong way to go.
But then I saw a '41 or '46 GMC 2-ton at the ATHS show in Shelton last week that had a 350 and an automatic with otherwise original running gear. The impression the owner gave me was that it is servicable if the demands put on it are like a 1/2-ton pickup.
The 450 series is a 3-ton truck, so this is heavy stuff you are dealing with.
