Can I cut down an OE biscuit mount if I need to?

I see a bubble centered on the index-mark.
I don't know if that's right or wrong.
Turn the level around the other way, and compare.Prove your level.
Plus I don't see what you are referencing.
How did you level the car?
You cannot use the wheel openings.Nor the fenders. They could be anywhere
I jack up the rear under the center of the diff. I put a round steel bar across the jack saddle, on the vehicle center line, such that the car is able to rock from side to side. Then I equalize the LCA inner pivot heights with the T-bar adjusters.Then a stretch a bar across the cowl, spacing the bar up as may be required, with equal spacers. Then I check the bar for level. Some guys check the side to side level, at the core support. If the cowl, or the core-support, is not level, prove the back is level first.Do not use the wheel openings. You can use the leafspring rear shackle upper bolts, to a level floor.If you shift the rear, you will have to readjust the the front also.When all 4 corners are so leveled, recheck the cowl/core-support.
After the chassis is proved level, then you can check the carb pad.
But, I suppose you already did all that......