In addition to what has been said above, if it is snapping, make sure the pump is seated flush and the shaft is straight . If the shaft engagement to the pump is not straight, there will be excess stress applied to the shaft.
Also be sure there is the right clearance between the shaft and the bronze bushing as well as the mesh with the cam gear to avoid binding and excess heat build up for lack of clearance and oiling.
Also be sure the shaft length is correct. For example if the block and china wall has been machined, you may have to shim the distributed or shorten the distributor tang that engages the oil pump drive to be sure the engagement and mesh of the gears is correct.
See discussion in this recent thread starting at post #146.
[URL]https://www.forabodiesonly.com/mopar/threads/old-race-340-teardown.501619/page-6[/URL]