GMC rods are cheap and plentiful. This swap began long before the insert rods were available for the Stovebolts, so they were used successfully in the pre-full pressure blocks. The GMC rods are also longer than the Stovebolt rods, so there are a few benefits associated with that as well for using them. You also have to remember that the average hotrodder back in the late 40's and early 50's was straight out of WWII and had a craft or trade skill they had acquired on Uncle Sam's nickle, most everyone had welding and machinist type trade skills. So doing something like that was no big task for them to undertake.
