Firstly, logistics...
I don't think you can
accurately drop a Toaster in the
exact Fender WRHB/Mudbucker spot, as the neck pocket is too deep from the 21 fret Foundy neck. My former Squier 72 Tele bass pickup had it's neck side coil located 24 5/8" from the nut, which is nearly butted up to the neck. My Foundy neck heel is 24 5/16" from the nut. It would probably be fine, but that's not a lot of wood between the heel and the pickup rout. A good ol' 21 fret Fender neck has the heel about 23.5" from the nut, which gives a safer feeling inch of wood between the heel and pickup rout.
For comparison, the T40 puts the neck toaster coils at 25 3/8" and 26 3/8" from the nut, which would work. My RicRiot has it's neck coil right at 25 3/8-25.5" area, which is where the '24th fret' harmonic rings out. A P bass has it's coils centered at 28.5", which is exactly where mine are on my Toaster equipped Foundation T. If you rout the neck pickup cavity towards the neck, you will have coils at 27 & 28" from the nut, which isn't far off from where a 51 P and J neck are.
If I were to do a Toaster located closer to the heel, I would put the neck coil at the 25.5" spot, which then puts the bridge side coil at 26.5". If you split the Toaster to the neck coil, it will harmonically sound sort of like a Ric neck pickup. IF you split to the bridge side coil, it will have a sort of a '7/8" north of a '51 P' thing. Just gotta measure three times, as routing for this location could run really close to the stock neck pickup cavity. If you fill the stock cavities, no biggie.
Now the tone...
The Toasters and Blades are not really present sounding pickups, especially compared to Super Ferrites. Chip Todd has commented that the T60 Toasters were better sounding bass pickups vs the T40 pickups, but the T60s were too narrow for the bass string spacing. If he could redesign the Toasters/Blades, he would give them more presence. I have noticed that my Blade in my RB45 is a little brighter vs the Toaster in the Foundation T. However, the Blade is still darker than a SF.
The best
sounding pickup combo on my Foundation T was when I routed the Toaster in the bridge and still had the stock Super Ferrite in the neck. The bridge tonality was thicker and smoother, the neck had that classic SF aggression and clarity. However, this was not the end goal, and I got the router out again when the 2nd Toaster showed up.
When the neck Toaster was installed, it certainly was smoother sounding (even in split mode). Something I could do to bring out more air and brightness to the Toasters would be swapping out the stock 250K volume and tone pots with 500K pots. I like higher value tone pots, and the 500K volumes would also raise the resonant frequency of the pickups. However, I probably won't do this. The stock pots sound and work great, and having a smoother sounding bass is a better choice for versatility in my bass toolbox-as I have plenty of basses that are bright (like the El Toro, Foundys/Dynas, Stingrays, J's).
Those Fender WRHB Mudbuckers usually had 1M pots to keep them from sounding too dark, but the WRHB (in series) is very powerful and dark vs a Toaster.