If your getting that deep the splines "should" be properly aligned. I had the same problem (3rd tranny, bad rebuild; bad junkyard; good one!) in my 68 C10. I was not quite aligned into the pilot bearing. After shifting the trannys alignment around (and cursing alot), it finally slid home. Just be patient, and don't get too rowdy while you try get it in. Those trannys hurt when you drop 'em.
