can you put true duals on a 2000 blazer 4.3 vortec 5 spd manual without loosing power?

Typically, no. The isolation of side to side exhaust flow will decelerate exhaust gas flow, creating its share of an EGR condition of sorts. I stronbly suggest the installation of a crossover, especially one of the X type. It will enhance low rpm gas flow without restricting high rpm flow. Be advised also that the sound of true duals on a V6 is not very pleasant. Ray