Your owner's manual has the refill capacities for the transmission.
As for the brake fluid, I myself would not bother with the expense of a 5.1 brake fluid, vs. DOT 4, unless I was racing the car on the track, or had experienced brake fade from overheating.
However, if you want to use 5.1, it is compatable with DOT 3/4, so typically one would bleed the brakes until the fluid in the reservoir is right at the bottom, gently fill with the new fluid, and then just keep bleeding the brakes until you see the new, clear fluid coming out of all four brake bleeders at the wheels. In my experience, a liter is more than enough to replace all the brake fluid in the system.
A power bleeder would be quite nice to borrow to accomplish this, vs. the cheaper vaccum bleeders or traditional pump the brake pedal process, which would take forever.
I don't think it's a good idea to deviate much from the recommended lube. Why do you want to put that heavy, goopy crap in your transmission??? Especially living in Canada! Talk about power loss! And I would worry about the effect the change might have have on your synchros.
There is nothing going on in your transmission to materially effect the chemestry of the lube. There's even a nice magnet to pull out any metal. Barring high water, the trans lube is easily good for the life of the car.
Ford's own transmission flush schedule for the MTX-75 is long...so long that I don't think they have a suggested interval for changing it. I've changed mine once, and that was because I had to drain the gearbox so I could take the axles out to replace the clutch. And yes, 75w90 is way too heavy...look into the Ford MTX oil or something more in the range of 5w-40 or so.
Ford actually uses a very good synthetic in the MTX, it is pricey, but nice stuff. Pick that up at the Ford dealer. The 5.1, I use it as well, same thing, if you are doing track days, why not for insurance. The motul is good stuff.