I vaugely remember that tab when I swapped to a poly trans mount.
If I remember correctly, I hammered a chisel in to hold it out of the way, then hammered the stud on the mount to beat it out of the cross member. I was frustrated for a bit by it too, but overcame it with brute force and a big *** chisel.
I know I didn't cut anything, and I'm pretty sure the poly mount didn't interface with the tab at all. But the poly mount did raise my output shaft a good amount (helped my header clearance too).