The open leg table gives unrestricted air to your systems return. You do not want to compromise the operation of your unit. I think even 6" would be to close. That is a mechanical area. The way to test it is to read the static pressure on the return side of the furnace. Let's say everything is properly sized and installed. With a magnahelic gauge or digital manometer you could read the static pressure on the return. Then move the heavier piece into place and read it again. If the static pressure increases you have restricted the air flow. I think your table looks great there;)

The builder doesn't create the ideal situation for everything when the leave an unfinished basement 'roughed-in'. We have an architech friend who knows the yahoo that designs the homes we're buying. While they are reputable, they do have a lot of design flaws - and these are greatly exaggerated when trying to finish an unfinished basement that is roughed-in/half done. There is a possiblity of moving the duct into the main basement, but it would mean creating ductwork that goes perpendicular to the joists, thus lowering the ceiling in that area. It may not be a big deal, but it's something I was hoping to avoid to save money (wasn't planning on hiring an HVAC guy when the basement gets done). The bedroom has one suppy vent - and the basement area has one also. My contractor has already told me I'll probably need to put in electric baseboard heaters, as that's not enough to heat a below-grade basement. And the basement finishing bill keeps getting higher!!!

I eventually had my HVAC guy come out and asked him. He said it was okay to partially block the heat vent, but if it had been the cold air return it would NOT have been okay. So I happily moved the frig to it's best place and moved the frig right up against it. So the heat vent is about halfway blocked and I just put one of those magnetic vent covers over the half behind the frig. I'm not an expert, but I think it helps to force more air out of the half that's not blocked. So, unlike most kitchen renovations, this cost me nothing. Yay!

It would appear that somehow your fireplace insert is tied in with you regular heating system. You will have to go do a little crawling around and tracing of the duct work and block it off where it needs to be. You have to be careful though to make sure it doesn't affect the rest of your system. If you're not comfortable doing it or not sure you're probably better off hiring someone who will understand what they're looking at in your house. It's pretty difficult to see into your basement from here.