Just bought my first mopar. Looking for help!

Run your VIN numbers and under hood tag numbers using this.
MyMopar - Mopar Forums & Information - 1969-1974 Mopar Fender Tag Decoder
My cousin had a swinger special that was a 318, if it has the tiny 7-1/4 rear axle it's a 318 car, '73 340's came with an 8-1/4 rear axle. And, welcome aboard!
BTW, a B/RB a is one of my lifelong dream builds, best of luck!