Yes, the RPK has a longer, heavier barrel. The AK receiver will not work to build an RPK on, the RPK uses a thicker receiver with bulged front trunnions and a different rivet pattern. Before you proceed, you should understand that AKs do not just bolt together like an AR. They require 'manufacturing' with special tools including a press of some sort, and riveting tooling.
Right now, that I know of, nobody is producing a complete RPK that you can order through a gun shop. There are several places that can build you one though, however you'll have to supply them with a parts kit and a receiver. And probably compliance parts.
You can buy mounts from
Kalinka Optics to put a red dot on, or you could just buy a Kobra, which is a Russian made red dot optic designed for AKs that fits on the side rail.