Your car is great!
I love most any Buick from 63 to 72. I'm currently working on a 64 Wildcat (I hate to admit on this forum because it has a nailhead in it).
If I were you, I'd keep the 250, keep reading, save some money and crank your 250 upto 250 or better horse. In that lightweight car, I'm sure you'll be pleasantly surprised.
Definitely do something with the three speed. I know your pain. My son and I just did about the same thing you your doing, except we did it in a 67 Chevy C20 (quite abit heavier than your Special). We ran it in the original form for about a two months before we started tearing it apart and, I have to say, the three on the tree was unbelievably AWFUL to deal with.
Absolutely no power on the highway until you hit 55 or so (several minutes into the acceleration), we were in disbelief that it was made with that trans and engine in such a heavy truck.
We put in a T5, and WOW, what a difference. Then we went through the engine and that helped more than the T5 did. I strongly recommend the 5 speed.
In case you forgot how I started this - that is GREAT car in great original shape.
