What do you recommend to upholster a pair of Girard chairs?  

Since these two chairs don't have removable cushions
then Case should think in terms of two fabric choices.
My suggestion is stripes or checks underneath and a coordinating solid on top. (all Girard and all Maharam)

i would pop the wheels o...
i would pop the wheels off and use them as pull up chairs, as a match and do them the same color. one wheel set, one stationary set, does not do it for me, unless you are using one as a desk chair and the other as a pull up chair then it would be perfect,
but it is up to you and where you are using these chairs.

LRF
I currently have both the chairs on glides and as they have height adjustable bases I have spun them right down so they're lounge height. They look really cool.
The guy I bought them off said originally they were from a sales desk and the one on the wheels was for the Braniff employee and the one on the glides for the customer.