Interesting question. I wanted to know the same thing but I can tell you the palstic ones frm crutchfiekd is a waste of time and may induce rattling. I had purchased a set of 3/4 MDF board brackets from a merchant to install some 6 1/2 in my Infiniti G35, the components never made it in the car, now I want to install them in the truck so those spacer I have I will use. Nothing against crutchfield, I've spent hundreds buying stuff from them including the said Polk components but those plastic brackets are crappy (thats why they're free). I know I haven't done you much good by not telling you where I bought the spacers from but thats because I forgot. Google 3/4 MDF 6 1/2 spacers and they should come up. GL
oh i didn't answer the second question. Most folks here say it's between 6 1/2 - 6 3/4 either way you'll be fine if you find the spacers i mentioned
