Try to find a 5235 instead, much better in terms of electronic than 5234 but also to 5234A. Crossover boards are compatible.
In Berga (who owns 4350 replicas) thread there is a link to an old thread I wrote on 5235, with crossovers schematics, formulas..
Sorry it is in French, I never got the free time to do the translation:
JBL 5235:
http://vintage-jbl.forumpro.fr/t3066...-jbl-urei-5235 (you can see pictures/measurements of a 18dB card for 4355 made using a JBL blank card).
JBL 5234:
http://vintage-jbl.forumpro.fr/t3212...-jbl-urei-5234
I can send you by email Eagle PCB files for 12 and 18 cards if you would like to build them or having them built.
I had recetly the opportunity to listen to JBL 4355 with a 24dB/oct LR active crossover, and it was perfectly working. Just try the three options 12/18/24.
You can do 12 and 18dB crossover boards and just by swaping them do the comparison by yourself. You can also start with your 5234 (do not modify it it will be easier for saling it after), and change later.