I ran an N20 cheater kit on my stock block 10.5:1 340 for yrs, way back in the 80's, from 125>250hp, never ever tipped a plug, 1 step colder, or had any misfires or anything, and only came back on the timing for the 250 shot a couple of degrees, we ran 98 oct fuel (4 star as it was called here). Ignition was MSD6A....went from 12.4 n/a > 11.01 w/250 shot, 6psi.
The 250hp shot then used 110/110 jets which I believe was well safe, I think the petrol jet is smaller on todays equivalent kits?. Therefore I would say it is even more important to run a dedicated pump and around a 1/2" line for N20 fuel alone for continued use especially with shots over 150hp, perhaps thats overkill, but thats how I had it set up and had no issues, (2 Blue pumps), ran same set up on the 440 6-pk bottom end car I ran as well, went down to 4psi as well a few times and went a touch quicker. Just some experiences from back in the day..........
340 was with a DC .590 solid cam, 440 was with a CC .650" solid