modernizing?

[Rodzilla]

so i have the chassis "clean" or at least clean enough to work on,i have had a peak inside and scared myself[if you ask me,the greatest invention of the 20th century was not the automobile or the computer...but the printed circuit board!...yikes ]

i plan to make a parts run in the morning...but am stuck trying to figure what i need to get with regards to the rectifier...i just keep reading stuff that doesn't really apply to what i need to do or contradicts other stuff i just read..so before i bust out my DMM and just measure the output voltage of the selenium feeding the B+...can someone tell me what i need to do to figure out what size resistor i need to install to use a silicon replacement??...is there an accepted rule of thumb that works for this?

i have just read on another board that i may not even HAVE to use a resistor at all,but just slap the diode in it's place since the filament supply is not taken from the rectifier and won't be affected..and the overall difference in B+ voltage will not be large enough to bother correcting[+/-20% tolerance]does THAT make sense?...it's been a while since i learned any of this basic electrical theory and i just need a simple answer...what do i gotta do here?