As far as “850’s” go, the street HP850(82851)is an odd carb.
It’s the only one I can think of that uses the 1.688 baseplate(instead of the 1.750).

It has the big venturi(1.563), but the small baseplate. This is the same configuration as the older “830” carbs.
I was never a big fan of that set up.

The original HP950 is the exact opposite. It has the smaller venturi(1.375) with the big baseplate(1.750).

In any case, since the Street HP850 uses the small baseplate, your Proform body will work on it.

Brawler race series sizes:
67199 650 = 1.250 venturi/1.687 baseplate
67200 750 = 1.390 venturi/1.687 baseplate
67201 850 = 1.390 venturi/1.750 baseplate
67202 950 = 1.450 venturi/1.750 baseplate

There are several configurations of various “850” and “950” carbs utilizing different venturi sizes and baseplates.
They don’t flow the same....... or behave the same.
It’s usually best to reference the physical sizes of the venturi and baseplate when talking “850’s” and “950’s”, to ensure everyone is on the same page with exactly what size carb you’re really talking about.

For example...... a few “950’s”
Holley 80496 - 1.375v x 1.750
Holley 82951 - 1.563v x 1.750
Holley 80805 - 1.600v x 1.750
Brawler 67202 - 1.450v x 1.750