Matt,
Too bad you aren’t closer to Indiana. I have a sawmill and some ash logs ready. I could cut you some lumber to order. No charge; it's for your church.
I sometimes have spalted maple as well, but I haven’t been able to saw it at the perfect time. The perfect time is after the log has sat long enough to get nice pattern all the way through, but not so long that it has started to lose all its strength.
Ash is relatively inexpensive these days. The Emerald Ash Borer is spreading across the country, killing ash trees as it goes. Landowners are cutting their ash trees now rather than lose them to the insect.
My guess regarding the look of the real cross is that it probably would have been hand hewn using flat-sided axes and/or adzes. The most common place I’ve seen lumber like this is in old barns. Most of those timbers are too big to use for a properly proportioned cross, but some of the bracing is smaller, sometimes 6x6's.
Check for old barns being torn down in your area. You could also do some on-line searching for sellers of recycled lumber.
Good luck,
– Loren
Baker 3667 portable sawmill. Cook's edger. Cutting my own lumber to build a kiln and a shop. Some woodworking & carpentry experience. Delta Unisaw & radial arm saw.