Yes, an older 68-76 block would be a good idea, since they will have solid main webs and are stronger. The late block can do 400hp with work, people do it to 403's with more rotating mass and the same windowed mains, but why build a windowed 350 when a good early block can be found for $1-200?
I would also say, if you are going the stroker route, why not get flat tops and go big block heads? Everyone agrees they are the head to do on a 403. I have them on mine and they work great. If you are coming into the upper 300 cubic inch range, why wouldn't it work the same for you? The only issue is that you will probably need to cut them a bit due to the big ~84cc chambers, but that's a cheap price to pay for the big flow, and they usually have the meat to cut them down a lot.
At this point, I think we are all still throwing ideas out, but why not look through some of the builds in that section of rop you posted in? There are plenty of motors in your power range, I'm sure there's a 350 build there doing what you are looking to do.
Also, I agree, don't forget to upgrade the rest of the car. You WILL NEED to upgrade the trans and rear if you expect to put down this power and actually have it last. My first 403 cooked a th350 and it wasn't that powerful (*maybe* 300hp at the flywheel). I then cooked another one cheaping out, not going to the right builder. I have had luck not blowing up rears, but traction has always been a problem, and that saves rears.
