Dear George and mid-60s Buick fans of . . . .
MORE!!!!What are your plans for this engine?
Probably rebuild it, nothing radical. Mild cam?
What do you suggest?
I noticed there are not a lot of aftermarket cams for the 300 motor, not even four barrel intake manifolds. Am I looking in the wrong place?
Anyone ever put a crate motor in their car? That's gotta cost a pretty penny.
I wonder if I am better off finding a Buick 350 or 400 - 
And, what about the V6 motor? I had no idea that they made a V6 back in 64-65.
Well do you have lots of choices . . . you just need to dig a little more.
Unfortunately, the 300 cid Buick engine is the least supported of the 1960s Buick engines even if it has a very proud heritage. It was the descendant of the all aluminum 215 V-8:
https://en.wikipedia.org/wiki/Buick_V8_engine#215As described on that Wikipedia article, Rover bought the rights to this GM engine and in 1967 Rover started using it. It was so successful that GM tried to buy it back! Rover used it for many decades. I have a Welsh friend who owned a Rover with this engine until a few years ago.
TA-Performance used to sell the Rover Aluminum heads that could be adapted to the Buick 300. I can't seem to find them right now though. Unfortunately, there is little in the way of speed parts for the 300. On the V-8 Buick there is this thread on the options to rebuilding the 300:
http://www.v8buick.com/showthread.php?191151-A-guide-to-building-the-lil-guy-The-Mighty-300You have many more choices if you consider the other Buick V-8 engines of the 1960s. When I decided to replace the engine in my trusty wagon, the big-block had so much more speed parts that it was difficult to say no especially given my goal of towing a heavy vintage travel trailer.
Today the Buick 350 has much better support and it less costly to build. I have a friend in New Mexico who upgraded his 300 to a Buick 350 and went to a lot of trouble to make it look like it was done in 1965 (right down to the nailhead green paint.)
You could even consider the later Buick V-6 engines that were in the muscle cars of the 1980s.
https://en.wikipedia.org/wiki/Buick_Regal#Grand_National.2C_Turbo-T.2C_T-Type.2C_and_GNXThese engines with turbo-chargers were considered the premiere "muscle car killers" of there day and there is a custom 1965 2-door Buick Sportwagon using that power-plant.
So you need to decide what you want out of the car. Simply rebuilding the 300 will make for an authentic car with some decent "get up and go" if you make the usual upgrades like dual exhaust. If you want
MORE you need to decide how much
MORE!!!Cheers, Edouard

P.S. Silly me I forgot the most obvious choice. You could also swap in an original 401 Buick nailhead and try to make your car into a 1965 GS clone. It is harder than the other swaps, but if you want that this is the forum to help you do it! There are some speed parts for the nailhead and it is a far more assertive engine than any small block. So that's yet one more option to consider. Certainly this is the place to look for a spare nailhead that is lying around.