Going to document this for posterity. Ended up knocking out all 4 plugs I could see. The three on the exhaust side behind the starter and the one on the end of the head (would be back on a tractor.) At first glance I was like, no way, no how on the side locations, as I've seen mentioned. More depth behind the head one, but no where for a heater unit to extend. Now I did verify my plugs were 30mm, and verified NAPA had 4 30mm plugs before I went a knocking ($1.01 each.) So I went looking for a heater. Searching the Kat's/5 star and Tempro/Zerostart sites, as well as discussions had yielded the Kat's 10417 and 11417 and Zerostart 3100009, 3100010, 310074 as possibilities. Ended up the only thing local (and this is AK folks) were Zerostart 3100009 and 3100074 at O'Reilly. Now I'm further going to note that I do not 100% believe the 3100074 I bought was actually what it said it was (could easily have been a swapped return.) No difference I saw between it and the 3100009 except a washer on it, and it being cheaper. It claimed in the O'Reilly manual it was 1000W vs 400W, but it is 34.5 ohm, so 400W (which is cast in it) and it claims 1 5/8" online at O'Reilly and others when it is 1 3/16" or 30mm.
So I get it home and start trying, although the head location is deepest, no way, no go, element sticks out beyond the 30mm a fair bit, isn't going to happen there (on a D750 factory head at least.) Try all the block locations that I figured no chance. Damn #2 location actually nearly goes. I take a carbide burr and gently relieve the block inside edge on the flywheel side of the hole, and just a smidge from the cylinder liner. Probably 0.015 from the liner, maybe 0.020 from the block. I take the heater and gentley bend the element out perhaps 10 degrees from 90. Slips right in. Against my judgment because I got worried about O-ring land engagement and my clearancing I install it with just a smidge of high temp RTV on the O-ring. Re-install the other plugs, put it together. This morning after sitting on 45 min on a 10F day the whole block warm and toasty to the touch. I ran the glows less than 30s (I used to hot wire them for 2-3 minutes) and it caught in less than 20 seconds.
Amazing... A rebuild averted for another winter.
So a couple caveats to those trying in say a D850, D950 or even another D750. Again I'm not sure this was the 3100074 heater (but looks right in the O'Reilly pic online.) I'm not sure if the OD on 850/950 liners is bigger or if the wall just gets thinner. There is NO way this would work if the OD's are bigger. Finally even in another D750 I would give this 60/40 at best that it works in YOUR engine. Be prepared that it might not. Only the middle plug will have a cold Kubota's chance in hell at fitting it.
Thanks all for the discussion anyway.