[Jack Gifford]

Glad to hear you'll be keeping the GMC Pontiac-powered!
Although mine isn't a 389, the '58 car engine is now 382 c.i. (4.127" bore). So my truck is very similar to what you are building. I'm using a HydroMotive HydroStick (built in a '55 Pontiac housing) which has manual control of all four gears, and a fluid coupling brazed and cut for 3,200 stall. I use a '57 Chevy car rearend with Posi and 3.08 gears. Instead of slapper bars, I designed a center torque arm (think late Firebird). With the 4.08 1st gear, rear housing control is a must! My tires aren't as tall as yours, but the 3.08 rear is perfect- I've run 75 mph for hours towing my lakester on a trailer.
I redesigned the '58 combustion chambers, eliminating the bathtub shape and reducing valve shrouding, and used KB hypereutectic pistons with D-shaped reliefs to get 9.05:1 which is happy on 87 octane.
As for carburetors, I love the simplicity of 2-barrels. My GMC has Tri-Power, but I've used a single 2-barrel (the huge Rochester 2GC from big Pontiacs) on other Pontiac-swap vehicles with milder cams than my GMC. It wouldn't surprise me if you'd be happy with a single big 2-barrel.