It is pretty cheap to change the Chevy pump pressure to see if it corrects the problem.
The chevy pumps of the 70's-80's put out around 1200 psi. The Ford pumps around 800 psi,thus you will fill the difference.
It is a common proplem with the Must II IFS to have too much boost & get
oversteer.
Do a search on Chevy pump pressure problem......

It is also important that you have the correct caster as mentioned above.