There is no formula. You used a chassis dyno, so the difference to FWHP is entirely dependent on your particular chassis setup. Changing tire pressure by 5 psi or changing the U-joint angles can give or take 10 hp. HP figures from a chassis dyno are only for comparative tuning on the same car and only generally relate to other cars - even with the same engine. If it makes you feel better, an average of over 40 random small block cars (Ford, GM, MOPAR, etc.) of 273 to 360 C.I. with aftermarket carbs, intakes and headers (otherwise stock) was 168 hp and just over 200 ft/lb TQ. I agree, your 289 would seem to fit right in, assuming the same mods.