With a small block Chevy in an early all steel Chevy, 50-54, doubtful if it would be B/gas (unless it's been really stroked to big inches) more than likely C/gas but it all gets down to the weight to cubic inch ratio.
I at first thought C/G, but the 357 c.i. is large enough to maybe bump it up to B/G. Many SBC powered '55-'57 Chevs were D/G, and were usually 283 or 301 in that class. Gene Schwartz's '51 Chevy ran a 9,000 rpm SBC that held numerous records in E/G! I have no idea what his SBC displacement was, but it was a heavy car I'd guess.
Here's the weight breaks for the 1965 rules:
Class ‘A’ – 5.0 0 to 8.99 # per cu in
Class ‘B’ – 9.00 to 10.49 # per cu in
Class ‘C’ – 10.50 to 11.49 # per cu in
Class ‘D’ – 11.50 to 12.99 # per cu in
Class ‘E’ – 13.00 to 14.59 more # per cu in
Class ‘F’ – 14.60 or more lbs per cu in
Based on a 357 in a 3100 lb. car it would bump clear up to A/G at the top end of that. But I think your 3100 might be a bit optimistic! Since those cars weighed in at 3400 lbs. and a straight axle doesn't really save weight over the stock suspension. Then add a rollbar, and ladder bars, and various other crossmembers, motor mounts, etc., to make it what it is. Might still make it into B/G, as 3500 divided by 357 would be 9.8 lbs. per cubic inch.