Dominic, all kiddin aside, rather than concern yourself with trying to make horse power, concentrate on making modifications that will build torque instead. With a low compression engine like that is, taking advantage of building torque will be easier. You've got 440 cubic inches. I would look at things like headers as you already have mentioned plus a good intake manifold. Everybody seems to gravitate toward the Edelbrock RPM for those, but they are expensive. If you're on a budget, you might can find an old Edelbrock CH4B intake. They were very good in their day and you can still find them reasonably if you look. Also, camshaft selection will be more critical with a low compression engine. The last thing you'll want to do is overcam it. I would recommend something in the .480 lift range no more than 230* duration @ .050"....and more like 226*....maybe something like .465/.480 218*/226* ground on a 112 or even 114 LSA. You'll want the LSA wide because that will make the torque curve very wide, including the bottom end rpm range right off idle. Also, you'll want to wake it up even more with a good loose distributor curve. You'll want total timing all in by around 2000-2500 RPM and it will probably like a lot of timing like around 38* total....but you can experiment around with that. And of course, yank that 625 off and put something more substantial on it. If you just have to stick with the Carter style, use an Edelbrock 750. A holley 750 vacuum secondary is also another good choice. It will make for a really snappy big block. It will seem like it makes more power than it does because of all the torque it will be generating. Of course, opinions will vary, but that's mine.
Oh and it wouldn't surprise me one bit for you to come close to 1 pound foot of torque per cubic inch with good parts choices and equally good tuning.
