The problem is I'm not sure where the drain plug is. I see the fill hole on the back, but where do you drain the fluid from? Is there another drain hole somewhere?
The Ford 9-inch rear end first came out in the Ford trucks and passenger cars in 1957. ALL '57 model 9-inch rears came with a 3/8"NPT drain plug in the bottom of the housing sump. MOST of the '58 & '59 model 9-inch rears came with drain plugs, but some didn't.
By 1960, none of the 9-inch rears came with drain plugs --only fill plugs located on the driver's side of the 3rd member casting near the input driving pinion. In later years, the fill/sight plug was located on the back of the rear end housing.
The 8-inch rear end came into production in 1962. None of the 8-inch rears were ever equipped with a drain plug from the factory. They only came with the fill/sight plugs on them and were located in the same places as you would find on the 9-inch rears.
You can try sticking a siphon tube in through the fill plug to vacuum the old oil out. However, with this method, you will not get
all the old oil out and if there are any metal shavings in the bottom of the housing, you most likely will not get
all of it out.
The best way to drain the oil, if you don't currently have a drain plug, is to pull the axles and remove the 3rd member from the housing. This will allow you to get
all the old oil out, check for debris or metal shavings in the housing, and to thoroughly inspect the inside of the housing, sump area and the condition of the 3rd member, ring and pinion gears, etc.
Once you have everything cleaned up inside, you could drill & tap the sump to accept a 3/8" NPT pipe plug and be assured of getting any and all shavings out before buttoning everything back up.
An 8-inch without a factory Limited-Slip/Traction-Lok differential will require (2) bottles of gear oil to fill it. Pour it in through the fill/sight plug until it just starts to run out the hole.
An 8-inch rear with a factory Limited-Slip/Traction-Lok will require 4 ounces of friction modifier as well as (2) bottles of gear oil. Pour the friction modifier in first, then follow with the gear oil.
This link mostly for 9-inch rears, but there are parts number references for 8-inch as well:
Replacement Parts