It seems that there are substantial differences between the two that would need to be addressed before it would work well it looks like.
https://tubedata.wernull.com/sheets/168/3/3KP1.pdf
http://www.r-type.org/pdfs/d5-100gh.pdf
Namely in the deflecting plates capacitive loads.
3KP1 seem to be around 8-11 pf, and d5-100 is much lower, .09-3pf? Hard to tell, not in English.
Also the deflecting plates voltages a a bit different, this is no doubt the source of the problem.