I'll try to keep from writing a book here, I need to get up early tomorrow. 
And I'll leave the EFI question to Matt. But a T-5 behind a Slant would be very nice, and fit the tunnel well.
My Barracuda is the same chassis as a '67-76 Dart. Barracudas from '64-69 were A-bodies. Wheel base was shorter on the Plymouth vs. Dodge until '71 then there was overlap (Scamp shared the Dart Swinger 111" wheelbase, Dart Sport shared the Duster 108").
I have a '68 Dart in my shop now. It's a customer's car. Stock 225, auto, 2.76 gears, manual steering, soft suspension and drum brakes. It's been a while since I drove a stock one. We're just fixing some things that never got fixed before the previous owner's husband passed away and the car was parked.
The stock suspension is soft. Makes the car feel much larger than it is. Quite comfortable, but not exactly my taste. The manual drums stop the car well, but leave room. Slow manual steering has good feel but you need to remember that you need to rotate the wheel a whole lot more to make that turn.
It's fun to drive as a cool nostalgic cruiser when you want to travel a little back in time going down the road. That's why he wants it. He also has an S2000 at home. The Dart would be a fine daily driver, but what people expect out of cars has changed a whole lot in the last half century.
My Barracuda is much more modern feeling. I installed much larger torsion bars (1" dia), front sway bar (1 1/8"), stiffer rear leafs (still experimenting for the perfect combo, but these are good), adjustable shocks, 11 3/4" front disks, and 10x2.5" rear drums. I'm also running a Firm Feel Inc stage 2 power steering box (15.7:1 ratio). My headers don't allow enough clearance for the longer pitman arm and idler to bump the ratio up a little further.
I've dodged cones with the car and run it on the drag strip. It handles very nice and doesn't beat you up. I've thought about building a 3-season A-body a few times. I can't bring myself to subject one to road salt. Not that they will dissolve, but if they have made it 40 years or more now, they deserve a chance to keep going.
As has been mentioned, E'bergs Green Brick in Mopar Action is probably the best known handling A-body on the internet. Rick has run in in One Lap many, many times. The evolution of the car has been chronicled in his magazine.
I'll only be accessing the computer at night for the next three nights. I'll answer whatever I can when I can. So any questions, ask away!