D911-
The OP has stated that the GPs are loop GPs, so they must be in series. The photo he posted also appears to have a ground wire from the last GP to the block. Also, I don't think the parallel pencil GP had been invented yet when his engine was made. So maybe that photo that you found is of an engine which has been retrofitted with pencil plugs.

To the OP-
Those pencil GPs were designed to retrofit into the same hole as the old loop GPs. So assuming your loop GPs are the same as later loop GPs (1960s), those pencil plugs would work just fine. But how many times per day do you start your engine? If it is only once or twice, I would say that upgrading to pencil plugs in parallel isn't really solving anything. Who cares if once or twice per day you save 20 seconds running the GPs?

If definitely sounds like you need to ream your holes. An earlier post of mine in this thread had a link to threads where I give the dimensions of the reamer. So if you are on a budget, get a drill bit of similar size and mark it to ensure you don't go in too deep. I will PM another option as well.

And adjusting the valves, as has been suggested, would be good use of your time. Good luck.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).