Dont use the 240 heads, use the 250 heads that came on the 1968 383 2bl they work great, get a purple shaft cam, the 3/4 one, problem solved.

1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5