I would be tempted to get the hydro-boost from a diesel truck, and eliminate the vacuum booster altogether. Finding those parts is prolly gonna be tough though, consider the age of the truck. I don't know what year range used the same system.... or if they just had a vacuum pump on the engine for the brake booster. Something to look into at any rate.

 

The 440 deck height is only about 3/4" over the 400. The motor mounts and brackets are the same. From what i found there are different boosters used but not sure witch one for a RB block.