Thanks to all who responded - some very good points to ponder...

I think I should spring for a different plate transformer, since I don't
want to be running this one too marginally. Also, I suppose I could do away
with the choke and still get under 1% ripple. That would definitely lighten
up the weight. The Hammond # 720 is rated 625-0-625 V at 300mA CCS and
500mA ICAS, so I guess that would be adequate.

I am using a free program called PSU Designer II to calculate the power
supply values and outputs. With the Hammond #720 and a 40uF/47ohm/40uF
C-R-C filter after the diode bridge, I should get about 1760VDC at 50mA and
about 1660VDC at 300mA.

I was also thinking of using three toggle switches across the front panel
for filaments, plates, and screens, arranged so as to reflect the order in
which to apply the power. I suppose some sort of relay or other device
could be used to make sure the screens never got power before or in place of
the plates. It is a good idea about the interlocks on the chassis too - I
know nobody else will be using this amp besides me but still always a good
idea to design in the safety. Also it won't be toured around as I play in
basement bands most of the time so it probably won't see too much road use.
It's more of a crazy way to generate 300 watts rather than go with the
ordinary 'three pairs of KT88 's' route.

thanks again sincerely for all the input!

Dave