Ant,
If it were mine, I'd get the G-Force T-5Z for a 5 speed.
With a big block, 9" rear, and slicks (and my son driving), I'd go with a toploader. I'm sure Dave K could build you one that was bulletproof.
With a 347, street tires and you driving; even ocassional track use, the T5Z should be adequate, and the G-Force T5 more than adequate.
I have a street/cruise car with a reasonably stout 351W roller motor. It's coupled to a Liberty blueprinted T5Z, Hurst shifter, with
synthetic ATF, has 15,000 miles on it now with no trouble since mile 1.
Should I upgrade the motor to a 393 stroker, I'd probably stick the T5Z back in untouched. If it started giving me trouble, I'd upgrade the trans to the G-Force. If my son wanted to do some track time with the car, I'd go G-Force.
I cut my teeth on 4 speeds in the 60's like a lot of us here did. The T5Z had an optional .80 5th that I had Liberty install. For highway cruising, it's terrific. Engine loafs along at 2300 RPM getting about 17MPG. The 2.95 1st is a little low and I tend to shortshift into second early. The trans shifts like a Honda; very smooth, and very quiet. A Cobra only weighs 2400 pounds, and with street tires, it's difficult to put enough torque through the driveline to damage the trans. The wheels will start spinning first.
The Tremec was a godsend to Mustang racers in the mid 80's. The T5's then, were only rated for 260 fptq, and were breaking left and right. The Tremec and T5 had similar 1st, and 5th gears which worked out fine in light of the stock Mustang 2.73 rear gears. The 5.0 didn't mind cruising at 1600 - 1800 RPM either. But the Tremec was, and still is a Mexican truck transmission. 3 of the 5 gears are usable in a Cobra. Over the past 20 years, I haven't seen Tremec responding to its end user base with better optional gearsets, nor has the aftemarket responded with more appropriate replacement gearsets. But the T5 has.
Today, I think builders opting for the Tremec, particularly for small block applications, even 400 inch strokers, are following the crowd rather than making informed purchasing decisions.
For mostly race, some street, limited cruise use a 4 speed toploader. For mostly street, a lot of cruising and highway use, ocassional track use; use the T5Z, or G-Force T5 depending on engine size and right foot size.