The 1965 and the '66 396 (and 1966 427 engines) had to have the groove to oil the top end. Without it your oil pressure will be blowing open the by-pass valve and your rockers are rubbing steel on steel without any lubrication or cooling oil for the rockers and the valve springs. 1967 and up the groove was machined into the block behind the cam bearing. Additionally even with a grooved cam it takes special 1965-'66 cam bearings with addition oil feed holes to line up when installing there rear cam bearing to feed both sides properly.
Here is an example of a flat tappet cam that has gone flat. Imagine the missing part of that lobe reduced to metal filings so small it passes through your ten micron oil filter running around in your engine pumped by the oil pump into every nock and cranny in the motor. Most get embed in the bearings and score the crank and cam journals (no lose with the cam as it is junk anyway).
That is why I asked you to check your valve lash again after adjusting them. If it is loose again after driving daily for a week or so it means that missing gap used to be metal that came off the cast iron cam lobe as BBC dose not use pressed in rocker arm studs the way a SBC dose (a stud pulling out is another source for a growing valve adjustment air gap).
Reason I mention it is that sounds like a fairly healthy aftermarket cam, and as such it probably has stiffer valve springs. Stiffer valve springs do a better job of controlling the valves, but that added pressure on the tappet going over the nose combined with the loss of ZDDP in modern motor oils generally results in a flat tappet cam failure.
Not just flat tappets suffer from lack of ZDDP. Modern LS Chevy engines are failing inside of warranty due to cam failures as well; and they use a roller tappet. It just takes longer for the wear to appear.
A pair of cams that failed in service that Chevrolet that were replaced under warranty.
Note the brushed look of the cam bearing journal above. The Crank looks the same as the metal is like sand paper.
ZDDP mkes yor rings and cylinder walls or any metal on metal wear point last longer as well.