Get an Eagle stroker kit, install Edelbrock heads, headers, a decent cam and you'll be over 500 hp. It will be fun on the street. I'm building a 56 now for a friend that has a 408 magnum stroker in it. By the way nice truck. I have a 5.7 in my 55 and it's a blast on the street. Smokes the rear tires at will.
For the poster wanting to install a 5.7, it's a tight fit. You will have to move the firewall back at least 7 inches. The 57 is physically a big engine. You will be giving up a bunch of leg room. I'm 5ft 8in and leg room is tight for me.