Good Evening, Russell,
General thinking is that enlarging lenses, even the good ones like your El-Nikkor, are best used at least a couple of stops down from full. If you use a condenser enlarger, your times for 8 x 10 from 6 x 6 negatives won't be very long with most papers. If you need longer times for better control or for some dodging, why not try f16 or f22. I'd bet that you will get good results form the El-Nikkor even down at f32. The theoretical problem with small apertures is not that the lens is any less sharp, but that difraction might become a factor. A few test prints at small apertures should be all it takes to find out.
Konical