Seized glow plugs

1980 Series 3 diesel
After deciding to fit a parallel glow plug setup I tried to remove the old glow plugs. Two came out easily (1 and 2) but the other pair (3 and 4) seem to be welded in place. I thought they were going to snap. Both were caked in carbon.
A friend suggested scraping the seat in the head, which is presumably what I call the shoulder in my diagram.
This makes sense because if the shoulders of the plug and hole don't mate well c arbon blow by will occur.
How do I do this? I made up a piece of hard plastic in the shape of the plug and scraped it round, and have been able to clean up the internal threads with a tooth brush but I am not convinced that this is doing any good.
Would a reamer help..if so what size? Would this remove carbon from the seat/shoulder?
Should I remove the glow plugs every so often to avoid this happening?
Compression is good on all cylinders. I use the LR about once a week for journeys of 15 miles.
Any advice most welcome.

Faber

You could also put a slit through the threads on old pigtail glow plug d use this to clean threads and also put a bit of copper grease on new plug fit and see how it goes , don’t think you’d yet any blow by

I found the old pig tail glow plugs to be better for starting than the newer ones.

Thanks for this. I am away for a bit and will let you know how things go.
Faber

If they work leave alone.