[Amps] Query: AC Switching of High voltage

[w2cqm@juno.com]

I'm retrofitting an Alpha 76A with 3cx800A7's. Eimac states
unequivocally, that "...heater voltage be applied for a minimum of 3
minutes before anode voltage and rf drive are applied...".  Rather than 
completely reconfigure the Alpha start up circuitry with their  step
start and  1 minute delay sequence, I've decided to  disregard that delay
function and add a 3 minute solid state delay relay and  to switch in one
leg of  HV from the  plate transformer.  Here's the questions: After the
delay, 

        1. Is it more prudent to switch  in one of the HV AC legs before
the full wave bridge rectifiers or  to switch in the         B-?

        2. Is a completely enclosed  mercury displacement* SPST relay a
better choice than a standard                                 P&B  DPDT
relay reconfigured for SPST to accomplish the task? 

* Can anyone  familiar with the relative merits of a  mercury
displacement relay  explain its  design?

Any guidance  would be greatly  appreciated. Ron W2CQM/3 

PS: If I could ever locate the elusive  SPST vacuum relay I have
somewhere in  inventory, the questions would be academic!