"i already decided to get a 3PDT to bypass the AA764's tonestack and 2uF/25V cap on pin 8 of 12AY7." You mean 12AX7? If you want to do a "switchable" hybrid I'd suggest keeping the NFB loop from the BF champ, just changing values. E.g. the 5F1 has a 1.5K Nfb load and a 22K dropping resistor. You could use the stock BF 47ohm NFB load with an 820ohm dropper, rather than the 2.7K fitted as stock. This would minimise the effect of the 12AX7 pin 8 cathode bypass cap shunting AC to ground when switched to 5F1 mode (a bypass cap will work here in 5F1 mode to a degree but not exactly ideal). Have the preamp bypass caps switchable.
The 25/25 cap at 12AX7 pin 3 on the BF schematic bumps up gain, this cap WAS actually fitted to most (if not all) 5F1 as well. 5F1 schematic is wrong/outdated.
Filtering - go 30-40/10/10 or try 30-40/20/10, 10's will sub for 8uf with no change in sound. No reason to go as low as 20 for the first stage.
As far as screen & preamp dropping resistors go, you might want to test by ear, perhaps go in at 5K/22K to split the difference, with a view to fine tuning? It's not unusual for tolex champs to run 6V6 screen voltage in excess of plate voltage with a 1K dropping resistor. Preamp voltage is obviously a factor, but so is the amount of dropping between 6V6 plates & screens, your ears will really tell you what you need, aim for 200v-ish on the 12AX7 plates as a start. Don't think about resistor values per se, think about voltage at the various stages & just use the resistors that give what you want.
47ohm at 12AX7 pin 8, folowing the cathode resistor - covered in earlier paragraph. It's the NFB load resistor for the BF rather than the 1.5K in 5F1.
Fixed bias is easily installed, or even a fixed/cathode bias switch (I have one). Fixed tightens response, less compression, less grindy harmonics. Won't make the amp vastly different but worth having due to the simple installation.
Other simpler route might be to go 5F2A with perhaps a switchable 500pf cap on the tone control, with the cap switched out & the tone pot up 5F2A becomes similar to 5F1. With the 500pf cap out the circuit and the tone pot at settings lower than flat out, you'll get a more "vintage budget" amp, darker, thicker sound. You can get a reasonable BF sound out of a stock 5F2A by playing with the controls, it's a pretty flexible design.
Another alternative might be to use the BF circuit & lift the ground connection for the middle resistor, treble pot becomes a tone pot, bass no longer functions & gain/mids are boosted. A 50KA middle pot in place of the BF 15K fixed midle resistor will do a similar thing when turned much past 12o'clock (measure pot taper to see what's going on).
Ignore the B+ voltages in the Fender schematics 370-410v at the 6V6 plate are more realistic, SF Champs might even hit 430vdc with the stock 470ohm cathode resistor.
If it was me I'd just go straight for the 5F2A with a 10"/12" speaker.