I had a naturally aspirated 289 with a '65 block, '65 cast crank that was indexed (throws trued to 90* apart) and the rod journals turned down to a V6 Chevy size, but not stroked, Howards aluminum rods, TRW 12.7:1 pistons, '69 351w heads with some port work, 1.6 roller rockers, Schneider solid roller (598/608 as I recall), Offy 360 Tunnel Ram with a pair of stock Holley 660 center squirters and it had a Jerico tranny with a 5.67 spool with 30x9x15 slicks in a 3000# (with driver) car that went 11.08@119.
From Tim McAmis Race Cars horsepower calculator....
Your 69 Mustang weighs about 3000 pounds and can complete a 1/4 mile in about 11.08 seconds. That means that you've got about 435.90 HP at the wheels, and about 566.67 HP at the flywheel.
I think the wheel number is about right, but I think the flywheel number is at least 70hp too high, I think he's figuring loss on an auto tranny, a Jerico doesn't have than much parasitic loss.
I think they start splitting at about 600hp.