While it ultimately depends on overall condition, mileage, options and such, a Slant-6 Coronet will not bring much on a used car market as it was not a very desirable combination.
anywhere from $2,800 to $4,100 depending on options.
I believe all R/T's with a 440 4-speed had dana 60 rear ends.
I assume your talking about plug gap as it's .035
go to the library and get a chiltons repair manual for your year car.
1-8-4-3-6-5-7-2 and the distribuetor rotates counter-clockwise
drivers side turn off to the left like normal lug nuts and passenger side turns off to the right.
pull the rear wheels off and look on the rear drum there might be 2 clips over the wheel studs, if the clips are there cut them off as they are not needed, if there are no clips don't worry about it. On the backside of the drum you will see 2 rubber grommets, pull them out and get a flashlight and look inside the holes and you will see a star wheel gear and a locking bar that rests on the teeth of the gear, you need to push in on the locking bar and then turn the gear downward as that backs off the brake pads from the drum, just turn wheel downward enough to loosen the drum enough to pull it off.
5 qts. with a new filter and 4 qts if you don't change the filter.
check the library for a chiltons repair manual for your year.
look on top of the brake pedal or on the master cylinder.
any 68 coroner R/T is rare especially a convertible.
If it's not under the dash on the drivers side look in the glove box to the left.
you can fit a 10" wide wheel under the back of the car with the proper back spacing but not sure if an 18" tall wheel will fit without raising the body some.
pull wheel off and then look for a slot in the brake drum on the front of it and rotate drum until you see the gear through the hole in the drum, turn the gear to set up the brakes.
Check out eBay or Hemmings or other collector car venues. A car like that isn't worth very much, pretty much only what someone is willing to pay for it.