Put in another plug,correct plug,if course,maybe someone dropped that one prior to installation and never evennlooked at it.. If a new plug does the same thing. Start it with that plug out,run it a minute,then put a plug in. If that plug closes too,that head needs to come off to see what's hitting the plug.Dont waste time trying to diagnose it through the plug hole the heads going to have to come off anyway to fix it. I've had plugs close up like that on other engines due to rod bearings. IDK if plug location on sbc will allow that or not. But that's on thing I'd be looking at. Incorrect rod bearing,wrong piston,wrong connecting rod letting one pistonbparkbto high in the hole.if it's doing this at cranking speed,as in never started,I wouldn't suspect a rod bearing,because there wouldn't be the momentum needed at cranking speed.