Hey-Hey!!!,
I think the L-C filter would gain us something, especially if a conservative rectifier was used. 400 volts @ 100 mA is 40 watts. 3A of 6.3 and 5 VAC is about 30 watts. 70VA core will have more to give the HV is full use of the heater taps is not made. It will not be all that close I suspect.Class A bias with 6V6's in U-L and using the middle taps and a 6BK7B driver pair( figure ~6 mA for that stage or so ) will give B+ of ~240VDC. No biggie....
regards,
Douglas