Looking at converters for a 727 torqueFlite. Some say front pump modification required. What's this modification about?

Here's a pic of the common pump mod, amounts to machining the area around the seal. The sloping cut is done at an angle of 25°.

the problems your bringing up is on mainly 8 inch race converts, maybe it is on some 9 inch also
As John has posted you may need to do some grinding or machining on the pump housing to clear the converter

I see, clearance for the smaller diameter converters. Thanks guys.