Even IF we had the technical capabilities in the late-1970's to have assembled the radioactive "engine" in space, the budget for the Voyager missions would have been blown WAY out of line with the multiple launches per probe.

> would’ve had to be assembled in space

> ...

> this all would’ve been very completable in 1977

first flight of the Shuttle was 1981.

I think you're vastly overestimating the ability of 70s-era spacewalks to assemble a spacecraft in orbit, including handling 600 pounds of radioactive material

Nothing is preventing you from building a new long-duration probe and launching it on a thousand year journey.