What year 5.0? Is it a roller motor? People get way more than 280 hp out of 5.0s all the time. That much I can answer. If this is a 5.0 out of a fox I would suggest looking around at some of the builds over on the fox body forum of the site. It doesn't have to be efi either.
It's time for me to decide on paint color for my 67 Fastback restoration. Will the value be higher if I choose a classic 67 color or doesn't it matter? I'd appreciate your opinions.
The K-codes were rated at 280HP back in the 60s.....of course, the rating system was different then, but the point is that it can be done with the 289 if you like, you have any number of aftermarket aluminum heads, cam, headers, and carb combos that should get you there.