If it's a 383, it's a wedge. That only refers to the shape of the combustion chamber. Basically, Mopar engines break down into 4 basic categories depending on the shape and style of their combustion chamber:
-Hemi
-Wedge
-Poly
-Flathead
As for whether it's a B or RB engine, if it's original to your truck, it's a B.
All 60's and 70's 383's are B motors. In the 50's there was another Mopar motor that had 383 cubic inch displacement, that might be considered an RB motor, but I don't remember ever hearing it referred to as that.
Check these links for more info on the 383's...
http://www.allpar.com/mopar/sonoramic.html
http://www.allpar.com/mopar/383.html
I'm positive the second one applies more directly to your motor.