How tight does the catalytic cumbustor on an encore 2550 have to be to the back of the refractory?

[Vermonter2011]

I'm in the processing of replacing my refractory and catalytic cumbustor on a 2010 or 2011 Encore 2550 (from the dark days of Vermont Castings). I replaced the cat on my prior refractory, but this one is very tight. I don't want to break the refractory, which seems to be holding up, but there's a tiny gap in the back now that I've pressed it in (temptation is to use a sledge hammer to get it in there). Is a tiny fraction a problem (less than a 1/4", probably less than 1/8")? My biggest concern is cracking the refractory or cover because there's too much space. I did crack the refractory cover once, years ago, when removing the metal plate to clean it.