Thanks to everyone who provided suggestions about low-temperature fasteners. I've always been careful to only use compatible materials for the wetted portions of the LOX tank, etc. but I hadn't thought about material properties of parts that won't see LOX but will still be cold. For the radial tank fasteners, instead of NAS623, I found MS27039C which has the same grip length but a little longer thread (ok in this case) and is made from A-286 alloy. Several sources indicate A-286 should be fine for cryogenic use and also has sufficient shear strength for this application. For the #4-40 cap screws used to hold in the seal washer, I can just use regular stainless cap screws.
-Bob
