Nothing is going to remove the "mail truck" sound, but I have to ask, just how much of a pig is it? Because a 283 is less cubic inches and a lot less low-end torque than a 292. That thing should pull like a locomotive, but it won't wind out, for sure. What do you expect from it, as an end product? Highway speed? Hauling stuff? It's important to consider. I'd personally roll it like it is, but all I'd be doing is hauling stuff from Lowes, etc., and not for too long a distance. NB- I have a fat motor truck with an automatic and all kinds of amenities, so maybe my "advice" should be taken with a grain of salt. It ain't the "Farm Truck", but I do have fun getting hole shots on ricers with four-inch fart cans.
Though I actually would like a truck to be used as a truck so that I can finish the inside of the '70 for short-term camping. The new linoleum I laid in is already torn from hauling rocks and other construction hardscape materials.