Screech at 70mph.

It very well could be the carburetor.
(Air noises from a gasket edge or something along that line)

The thing to do is narrow it down via symptoms.
Tighter or looser belts will change the noise.
A bar of bath soap touched to the running belts will silence a belt squeal.
Note throttle openings and loads that it does it, and if only in one gear at one speed and no other.
I realize that is what you said, but are you absolutely sure as you can be that this is the case?

Only at one speed and one load sounds like a carb noise to me.

(Unless it's the speedometer) :D