Never checked the difference between a regular and a short filter. But the official rating of a stock bronco V-8 pan (for all years if I’m not mistaken) is 6qts.
If you use 5 1/2 quarts with your small filter I’m sure you would not be improperly filled.
By the way, what is a small filter? What size, what part number, and got pics?
Was your engine built around the use of synthetic oils? If not I would wait for maybe 5000, or even 10,000 miles before switching.
Yes, many modern engines run with synthetic right from the factory. But in the old days the rule of thumb was not the initial run in of 500 to 1000 miles, but instead giving rings and things the time to fully seat, such as 5000 to 10,000 miles.
That’s still my practice anyway. I’m sure you’ll get very a wide variety of suggestions.
You can run anything from a 10w/30 (or even 5w/30 in a colder area) to a 20w/50. I happen to like the 15W/40 that you’re already familiar with. I’ve read a lot of threads here where people with the thinner oils such as the 10W/30 were commenting on the oil pressure being lower than desired. Even on a fresh rebuild.