Why scrap a perfectly good 350? If it is the at 69 block it has different, more solid build than a 74 smog era block. As mentioned, a pontiac block is a pontiac block, the 455's have a different sized crank journal than a 400. Depending on who you talk to, punching out a 400 or 350 to a larger size is preferable, as you are keeping more iron around the crank. More iron, more RPM. I am far from an expert, but my 400 block has been in engine jail for 3 years (getting tired of waiting on it). It'll come back as a 462. .035 over with a 4.25" stroke.
I pick up any 60's pontiac block I see if it is in good shape. Posted a while ago about a cool one I found with factory 4 bolt main's that was in a Bonneville. Great find if I really want to build a monster.

If I were in your shoes, I'd pull the 350, strip it down to the block, have it machined and cleaned, and build it fresh. You've got the sweet sweet 4 speed, so putting a nice cam in it and some high flow heads will really wake it up (it's not that simple, but it a little research turns up a lot of info).

Hokie... I'd have bought that 326 all day. Those things will rev forever after a good rebuilt. I have a C-20 with a 327 that runs to 7000 rpm after a relatively inexpensive rebuilt.