That Paxton looks pretty straight-forward as far as install. I think 400HP from a 273 is probably very doable, though like others have mentioned, I'm not sure how much PSI it would require and what kind of fuel it would take to keep you out of detonation with those 10.5:1 pistons. If money really is "no object", I would consider pulling that 273 (after I paid off my mortgage and bills, lol) and saving it in the corner of the garage and replace it with a more boost friendly small block like a 360 or even a 318. Nothing rare you would cry over like your 273. Best of luck with the project. Will make a super sweet sleeper.
