Early fuel pump [front distributor engine] Short arm, will fit but will not work in late engine.
Late fuel pump [rear distributor engine] Long arm. Can be made to fit early engine but arm will break after about 1 to 2 miles [1.6 to 3.2K]

Fuel pumps will push about 4psi of pressure.
The engine can produce about 21 inches of Mercury Vacuum at the inlet manifold without much problem when in good tune. [not 21 pounds of vacuum]

If it's rich, then possibly the needle & seat are leaking or float level is incorrectly set.
If it starves for fuel under load [high fuel demand] then the first thing to check is the fuel filter. These things prevent rubbish from passing through & therefore slowly block up eventually causing a severely restricted fuel flow.
Replace the filter as a first measure. It's cheap & easy to do & could save a lot of grief.

Have a good look inside the old filter to see if the filtering element is still glued to the filter body or if it is detached & rattles around.

The alcohol in E10 fuel slowly dissolves the glue holding the filtering medium in place & if it is detached then it's possible that a tiny piece of rubbish has made its way under the needle & seat to cause low speed flooding while a larger piece may be partly blocking the fuel line leading to fuel starvation at high power settings.

There is also a small gauze strainer under the needle & seat assembly & this might be partly blocked.