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.
I’m not relying on the radiation to power it.
But rather, it’s own decay process.
That’s what they used.
The slower it decays, the more you’d need,
But the longer it decays, the longer it’s powered.
Which is why plutonium seemed so perfect.
But, we simply would’ve needed a little more time,
And it’d have been done,
With radium.