You don't need 3" pipes in a truck! I also have a '59 Apache with a 350 engine. I 'tuned' the pipes to a specific RPM level (3500 RPM) by using the diameter of the pipes to build a reverse sound wave to 'extract' exhaust at a given RPM for a given displacement. I use the formula:
1" of diameter per cubic inch of displacement for 7000 RPM for the whole engine. I use my engine at 3500 RPM max, therefore I only need half the calculated value at 7000 RPM. Therefore, I wouldn't go over 2" diameter pipes if I were you.
Regards,
