R43 Spark Plugs don't fit in crate engine?

[saxart]

I just tried to install some AC Delco R43 spark plugs in the crate engine (190hp 350SBC) in "Old Roy" and they won't thread into the spark plug holes. I had a set of older (used) AC Delco 41-905's (tapered seat) and they went right in. I'm guessing the R43's are the wrong plug for this engine? Should I run a new set of the 41-905s? Or something else?? For what it's worth, I'm running a Skip White HEI distributor and plan to gap the plugs @ .45

Thanks!