The Olds 350 is almost a direct swap. I say almost because the 307 had a very thin balancer and the 350 has a normal sized thicker one. This will cause some issues with the pully's and belts lining up but I don't remember what the simple fix for this is ( but there is a very simple fix ). If you can find an entire set of pullys/accessory brackets from a late 350 or 403 and use them, I know that'll work.
On my 85 442, the 307 had a normal sized balancer to start with, so when I swapped in my 71 350, everything bolted right up ( including my 307's intake and exhaust manifolds ).
Now the A5 intake from your 307 will fit your 350 but it has very small runners/ports and would be a big mismatch on your 350. In other words, don't do it. If you're going for the stock look and/or don't want to spend too much on an intake, get an A4 intake from an 84 or earlier 307. It will look identical to your A5 but won't have the port mismatch problem. I have a nice powder coated ( semi gloss balck ) A4 for sale if you want it.
You're exhaust manifolds from your 307 will also fit on the 350 but like the intake, would be a big restriction. They really suck. You need a set from an earlier 307. I have a very nice set of these if you're interested too.
I'm not telling you you 'need' this stuff so you'll buy my crap. I'm telling you you need/want this stuff ( whether it's mine or not ) because you DO. An A4 and early 307 exhaust manifolds should be easy to find in a junk yard. If you're interested in buying my stuff, just PM me. I'm not looking for alot, I just don't want to store them anymore...
