[Amps] Anode clearance and Chassis WANTED

[millenniumfalcon at cableone.net (Jim Isbell)]

I am building a 4-1000A linear and will have 4KV on the anode.  What is 
the MINIMUM clearance I must have from the anode to the top of the metal 
cabinet?  It looks like it will be about 1" in my present configuration. 
 Will that be sufficient to prevent any problems?

I am looking for a chassis 14"x13.25"x3.25".........may have to go to a 
shorter one if the answer to above question is NO but really need the 
3.25" for the input tuning box to fit.