I would see if the engine is salvageable. You can seize an engine without destroying it.

If it's not, find a running 400, swap the heads, but put a milder camshaft in it. A Summit 2801 would be a nice little camshaft at a budget price.

The best thing to make it streetable is to really gear it down (which will work better with the milder camshaft). The lower numerically, the better. Something in the very low 3s would probably be ideal.

Keep in mind, even with a Chevy engine, you'd still need to change the 4.11s. And swapping a Chevy would require new frame mounts, new headers, a trans adapter (if yours is an auto), not to mention it would need to come with all the accessories intact and everything. Just keep it Pontiac.

One more thing, perception can be everything. Ditch the Flowmasters (especially for a daily driver). Not only are they loud, but they tend not to flow as well as other (even quieter) mufflers. Throw some Dynomax Super Turbos on it. It will still sound nice, but take enough bite out of it, that mom will feel a little better.