What would be the 'best' supercharger for my 1970 C-10?

in all honesty you may have too much compreaaion already to just bolt on a blower. with milled heads and being unsure what cam and valves and such wouldnt be willing to say for sure but generally a low compression motor is built to be blown because the blower is essentially a compressor in itself. you should talk to a engine builder not the ppl on here in all honesty a trusted builder of performance engines