What size header bolts for a 72 340?

I use allen head ones for all but "those" hard ones (you know the ones), for which I use regular (hex head) bolts and it's easy to get an open-end wrench on. But for the frontmost and rearmost on each side, I put in studs. You can seal them up good (water passages), but also it makes install so much easier, especially if you're installing the engine yourself. Just wrestle each header into place, letting the studs hold it on, then start putting the bolts in. Makes a hard job much easier. I didn't have to dent anything on my headers, but then, every kind of header is different. The most recent ones I did were Hooker Super Comp.

The other maybe not-so-obvious advantage to using studs on the front and rear holes is that if you need to change header gaskets or whatever with the engine in place, having those studs there keeps the water in. I really like the Remflex gaskets.