Well yes the old style Narrow Bolt Trans Pattern Bell Housings for sure never existed in any of the 1980's Mustangs or for that matter any of the 1967 up Mustangs or any of the many other 1967 up Ford & Mercury Model's which by that time all would have used the wider Trans mounting.
The last narrow Bolt Bell Housing made were in some of the 1966 year models. In the case of finding a SBF V8's its only a narrow window of the first 289 6 bolt Bell Housings that were made after August 3, 1964) - these had 6 bolts holding bellhousing to block for use on the rest of the 1965 year models using a 289 V8 and probably into the first half of the 1966 year model transition up until all the supplies of the narrow mount transmissions were used up. The change was made for all the 1967 year models using a 289 V8 as well as all six's and other engine families to the wider transmission mounting. See below first two pictures for the 1961 to August 3, 1964 SBF V8 Bell Housing all of witch had the narrow Trans Mounting Pattern (they were the early 5 Bolt bells). Last two Pictures is of a late 1966 240 / 300 F100 Ford 6 Bolt Bell Housing. This was also a transitional Bell of sorts because it still has the Undrilled and Tapped Bosses for the Narrow Transmission Mounting Pattern if you look closely at the last picture of the back side you can see the top bosses are still cast there in place, it would be easy enough to place your trans on that bell and mark the holes so that you could drill and tap those bosses and then you could use your 3.03 trans on it. That Bell is for sale right now on eBay here is the listing.
https://www.ebay.com/itm/154813419484?hash=item240b994fdc:g:cC8AAOSw4mFh8b6w&amdata=enc:AQAHAAAAsF38pALxj35KP8IvoJYKs5gLiUpmKT0kuqjH2dAI1YsfsAzsUubfXQNwEoypGDxBIqIZtvakP1s6e5GV8NwzZzCMszqftp57LXBfZxvsI6cyxLfNktpCM3EGcfwj796p2pT99IwUBSsXpdAMMOb19C84ld4tv/6hVsitQjc/7HrmZ8CcmX79IxOZJyWWIsrWoQ5SsM00t0Mp6hwbfHxENduuLMw6PLSgbCn9DXqXGk4N|tkp:Bk9SR4b3_vDhYA
I have done these type Mods many times over the years when I couldn't find the right part since it was so much harder to find. Also if you can Weld Aluminum or have a friend that can, than you could also use those later 1967 up wide bolt Pattern SBF V8 Bell Housings that seem to be everywhere. They could be welded up on the back side to make thick enough Boss for you to then drill and tap the same way to get your narrow bolt Pattern Trans mounting. Best of luck