The 75 jets with 61S metering rods sounds like a big block 454 motorhome application. Those carbs are PLENTY with the stock factory 75 jets, and the 61S rods are full taper.
The stock 75 jets may actually be fine, use the APT system to fine tune the A/F ratio at part throttle.
Do not compare what is found in the earlier q-jets like the 17057202, those carbs have a completely different main airbleeds set-up and are pretty lean even with 77 jets in them.
A single plane intake and adapter to spread bore is also a poor choice for an intake set-up. Not sure what camshaft is being used, but single plane intakes are almost always poor choices for small block applications, as they lack low end torque right to start with, not to mention the miss-match you get putting an adapter on one to spread bore......Cliff