Spacers should be inserted with firm thumb pressure. One of the things that impressed me when I watched the process of re-working a PI 390 block to take cross-bolts---At one of Roush's facilities---was that the tech insisted that all the parts could be assembled with no more than finger-and-thumb effort. From the factory, the assembler had three different 'lengths' to choose from. I don't remember what the difference was---if I ever knew. If you can get them in without the use of a hammer and they don't fall out when you let loose, you'll probably do just fine. I have two of the ones we made because, although we had a complete set, you'll want to remember that this was a 390 block and although it had the 'knobs' cast in place, we had to do the cutting. FWIW, the two extras I have here are---well I had them in my hand the other day---here they are---.3732 & .3735.
The original 427s, with factory mufflers,had a slight 'whirr' to their idle sound. And please remember that no 'Y' block had anything other than solid lifters. If you were looking for it, you could hear the slight sound. Otherwise, not.
KS