The old 6 cylinder most likely weighed MORE than your 350 SBC with the aluminum intake and tube exhaust. Plus the SBC is further back than the longer 6 cylinder. I have done the 6 cylinder to SBC changeover many times in 55 to 57 Chevrolet's and a few 68/72 Chevrolet Novas and never HAD to change the springs. IF ... it sits like you like it ... I most definitely would not change the coil springs.
"I won't be wronged . I won't be insulted, and I won't be laid a hand on. "