When I rebuild my drill battery pack with new C nicads
do I have to stay in the same mAh rating. The originals
are 1300mAh, I can replace them with 2300mAh cells
with tabs. Or even 4000mAh ones. Will more mAh hurt
the drill?

Better question, will it charge with the stock charger?

It should, unless the stock charger /required/ special cells for rapid
charge, or some other condition.

Using NiMH cells should eliminate any problem, as they (generally) tolerate
rapid charging. I used regular NiMH cells for a Polaroid #365 electronic
flash, and the #363 rapid charger handles them fine, even though it was
designed for rapid-charge nicads.