The Autolite 133 is a .460" reach, non-projected, tapered seat plug, which crosses to an NGK R5673-8 (3249)...so that should be an exact match. Did you index the plugs by any chance? Also, I was thinking the 186 Z heads used a gasketed seat plug, no?
Here's the chart:
click here Do I have the seat type wrong on the small block stuff?
wheelhop: we run the R5671s in all of our 840 headed stuff, and the engines love them...you could try a GR5, which is the standard plug version of the R5671, except it's gonna have the extended tip.