How much RPM drop from park to drive with stock type 340 convertor

We put a 340 with a 904 in my sons Dart, stock engine with 4 spd type cam. The trans and convertor were bought used. The convertor was supposed to have a 2500 stall. I tried to find a number or something on the convertor to verify but no luck, and I didn't think to measure it at the time. It took 7/16 convertor bolts, so I know its not a factory one. I figured stick it in and see. Well the trans seems to work fine but the convertor seems real tight. I get a 300-325 rpm drop when it goes in gear and its definitely not a 2500 stall. What kind of rpm drop would I see with a stock 70 340 convertor? Thanks.