Yes you can swap in a T-5 relatively easily. I don't think it's a direct bolt up. The mounting holes on T-5 have to be enlarged, and you'll have to run a different clutch disc (depends on number of splines). While you're in there you might as well replace the entire clutch, unless it's new. I'm about to do the same thing with my '68 and 250. I got a T-5 from a '91 Camaro which has a .63 overdrive. I'll have to overcome the shifter issues, but it'll be worth it. Take a glance through some of these links and I think some of your questions will be answered.
http://www.thirdgenresource.com/transmissions_information/
http://www.stovebolt.com/techtips/t5tranny2.htm

Also go to www.inliners.org On the left side you'll see a list. Click on "Tech Tips", and then you'll see another list. Just click on the T-5 page. Hope this helps. John

1968 Chevy C-10, 250 and 3 speed on floor, Hedman headers, Offy intake, Edelbrock 600cfm 4 barrel, HEI
1966 Chevy C-10, motor and tranny went into the '68. Waiting to be restored.