Just had the top end of engine apart.
Now when the MSD cap is on( has the two spring loaded dogleg deals on it to fasten it…it’s not tight, can move the cap around with the legs out.
Is there some sweet spot, register I am not seeing. Getting an intermittent loud backfire, thinking this cap not seating correctly has something to do with it.
Don’t think it should move around at all. Has the standard push lock type non HEI cap on it.

See the register now, dark in the bay where my car is, no lights, and I couldn’t pull it out to see better. Locked customer car is blocking my door right now…

depending on which version distributor you have, early Chevy type cap or later screw on cap.
The one you're talking about, I think, should have two slots on the base of the distributor for those spring loaded hold L shaped downs to fit into

There is two l shaped spring loaded fasteners with Phillips head screws. Should be a little tooth on the cap to mate up with a slot on the base of the dizzy, but there isn’t.
This may not be my issue then because the cap was on my old combo and it ran fine, even though the cap doesn’t fit tight and will move around if you grab it.

Edit..this must be some kind of generic knockoff cap. No tooth/ locator.
Looked on Summit racing at the correct cap..MSD 8437, and it indeed has a little tooth on it beside the window to fit in the notch of the distributor to lock it in place.
The fact that the car previously ran fine the way it was tells me this probably isn’t the issue I am having with a very intermittent backfire/ loud pop, after heads and intake swap
Darn

Put a tad more arch in the clips to effectively shorten them.
This will tighten the cap to the housing.
It probably won't correct a misfire , but all the MSD caps I have had used a tab and groove kind of deal to orient the cap , and it would take a good deal of effort to put the cap on wrong.

My cap doesn’t have clips. It has two spring loaded L shaped little arms. I found out the problem. This cap is a knockoff cap, not an MSD. It doesn’t have a little tooth/ register on it that the MSD or Mopar performance caps do, that seats into a little notch on the distributor. This little tooth prevents the cap from moving. Locks it in place.
I ordered an MSD cap, will have it this morning.