The length of the overlap actually depends on the length of product starting from C primer. So for 750-base long cDNA there will be ~50 overlapping bases. Distribution of overlapping bases is the derivative of the length of products.
As for the primers I’ve just got the reply:
We don’t have primers to reliably tell IGHGX, as the primers should be placed ~70 bases downstream of J and they are not so reliable (only 2 base difference between G2/4) so we decided not to focus on those in current protocol.