No experience with the 347, but I had a 1958 336 in my 1959 gmc when I got it. The 336 is basically the same engine as the 347. It was a very reliable trouble free motor. I have since replaced the 336 with a 1959 pontiac 389, which I have had bored and stroked to 447 inches. 1955-1959 pontiacs are vitually interchangeable as far as dimensions and mounts go. A 1960 pontiac will also bolt up to the hydramatic transmission. The timing cover is different on the 1960 Pontiac as the 1960 motor is not "reverse cooled like the 1955-1959 pontiac and gmc v8 motors.
Here's an interesting link with hp and torque specs.
http://enginefacts.com/pontiac347.php
