The 'wide block or Poly' is a good motor, so keep it. Don't take a perfectly good Mopar motor out of a Mopar truck to put in a Chevy, [gross].
Chrysler wanted to water down their Hemi V-8 overhead valve motors. The motors needed to be lighter, cheaper and easier to make than the Hemispheric combustion chamber headed 'Hemis'. The Chrysler engineers came up with the polispheric combustion chamber heads so they could use only one rocker shaft and still have canted valves. On the 'Poly' motors the combustion chamber is multi-angled and not quite hemispherical and the valves are not quite canted away from each other as much as the 'hemi'. The rocker arms are on the same rockershaft, but the intakes rock one direction and exhausts pivot the opposite way. These Poli's aren't quite as good as a Hemi but they're real tough and pretty strong runners.
Look up PolySuperPac for headers, fourbarrel intakes, racing cams, and valve covers.
Nobody in their right mind takes a Mopar out of a Mopar to put in a Chevy.
Nice truck.