X2 with blykins. Too high compression ratio for gas used. Advance to high for compression. It does not take high rpms to damage the engine under these conditions. With loud exhaust it makes it hard to hear it knocking. Since you have some pistons out measure or find the CCs of the heads and find or calculate piston valve relief CCs and piston height. Do some reading on Dynamic Compression ratio. If you have the cams specs you can calculate it here.
https://www.uempistons.com/index.php...bc6726aec853da
Generally, you want dynamic compression between 8 to 8.5.
Static compression does not take into consideration your cam, which is critical.
I have a friend that can't drive his Cobra on the street without race gas. Big mistake on the engine design. Good luck.