This doesn't actually look like any of the Fender to Marshall mods I've ever seen. Usually they include changes to cathode bypass resistors & caps, changes in plate resistor values, even an additional gain stage.
I would question the practicality of a cheap, simple mod like this, that then makes your expensive (in comparison) speakers obsolete (assuming that they sounded OK before the mod).
Both the 5F6A bassman and classic Marshall circuits use a cathode follower tone stack, after an additional stage with a volume control - which sounds different to the plate fed tonestack that you have. In short, I'm not sure that there's a simple DPDT switched mod that can get your Normal channel in that territory.
Being more realistic, I would be more inclined to mod the normal channel to a more Marshall inspired channel (as you have, but with fixed treble cap & slope resistor - values decided by listening tests) and maybe have the 2nd stage cathode bypass cap for that channel at .68uf and use your switch to switch between the .68uf & 25uf. For this, you will need to provide each cathode (pin 8) at both V1 & v2 with its own cathode resistor (1500ohms at V2, 8 with a 15uf/25v bypass cap and perhaps stick with 820ohms at V1,8?).
For the cathode bypass cap at V1,3 try 220uf, 100uf & 25uf and see what you prefer, A/B a 2.7K cathode resistor against the stock 1.5K also.
Depending on your preamp voltage (I'd expect to see around 220-235v on the preamp plate of a 5F6A with 12AX7 in V1), larger power supply dropping resistors (in the cap pan) may also be in order.