350 Buick has a taller deck so it's a little bit wider and the exhaust manifold is a different shape due to the changed valve order, but i don't know that that will make any difference.
Of all the swaps this one is one of the easier ones.
I did this on a 65 Skylark IIRC the pulleys and brackets from the 300 will bolt straight on,
wire harness will also work.