I've noticed highly radiated areas contaminate modded logs and grass plants but not grass blocks, dirt and stone from other mods.

It also doesn't affect vanilla gravel, but I'm not sure if this is intentional.

Maybe sellafite can be a default block for replacing contaminated blocks.

I've noticed highly radiated areas contaminate modded logs and grass plants but not grass blocks, dirt and stone from other mods.

Yes. It's because we don't have unification for other grass.
It's simplier for log because alot of modders just add new log based on default log from minecraft. It's like ores and other.
It's fixible for grass also. But a tiny harder, maybe.

I'll see what I can do