Parts list shows #8 screw with #6 head but your measurements indicate #6 so best to go with that

The slotted oval head screws will be hard to find but Restoration Specialties has Phillips head screws.

Part number AV2711, #8 x 7/8 with #6 Phillips oval head, chrome

The cup washers would be #6 flanged countersunk washers, available in nickel plated brass from R-S, Part number AV3469

Original rear wheel cylinders were Wagner FD2863 and FD2864, L/R with 15/16" bore..
These used a brass fitting to adapt to the inverted flare on the steel brake line.

It looks like the baffle plate from the bottom of the oil filler tube.
If the tube is installed wrong the baffle plate can be hit by one of the crankshaft counterweights and damaged.

Circumference in the 51" range is still too short.
It needs to be in the 54" range.

Online search found Gates TR 30537 available at reasonable cost from Rockauto and probably Napa as well.
Specified as top width of 7/8" and outside circumference of 54-1/8"

I checked some old original belts and found the following measurements for top width by outside circumference :

Gates 85-T
One at 7/8" x 54-1/8"
One at 15/16" x 53-5/8" (snug fit over only crank and fan pulleys)

Modac (Old NAPA brand) 25-13530 at 15/16"x 54"

I also have a new Dayco 34500 that was purchased many years ago as a replacement for Packard # 395959 used on 1948-50 Custom 8 and found that it is too short and will not fit over only the crank and fan pulleys.
Measured that one and found it to be 1" top width and outside circumference 51-3/8"

The rear bearing for the transmission main shaft is retained by a snap ring in the front side of the adapter housing and is virtually impossible to remove with the assembly in place, so the complete main shaft and gear assembly needs to be removed.
This can be done by driving out the countershaft and dropping the cluster gear to provide clearance for removal of the mainsheet and gear assembly.

The 1935-41 and 1942 parts lists both show spring #354691 for 160 and 180 5 passenger sedans, load 1990 and rate 100
Bear in mind that the data book was printed before production and the parts lists show what was available to dealers from the factory for replacement items, not necessarily what was originally installed.
I suspect that the 1941 160 springs should work just fine for your 1942 180

There should be 2 numbers stamped in a recessed band in the shaft near the drive end of the gear, one for the Packard part number and one for the Stewart Warner number.
This will give you a positive identification.

Love the stooks in the field behind the car at 1:43