Furnace venting inside a wall

I am remodeling a bathroom. When I ripped out the shower stall I found behind the wall is my furnace vent pipe which is 7 in. diamter Type B gas vent pipe.. So rather having just a stud wall, the wall behind the shower was framed seperately 5 inches out from the framing of the other interior wall to allow enough space for the pipe plus the 1 inch gap on each side that is required clearance from combustable materials. I would really love to regain that 5 inches of space to allow for a larger shower. Can I just replace that section of vent pipe with an oval pipe that is designed to go inside stud walls?

I have not seen 8" oval. Amerivent seems to have only 4,5,6

I screwed up, it is a 6 inch pipe. So can I just change it to a 6 inch oval?

Actually, the vent connector coming out of the furnace is 4 inch type B pipe. For some reason it then changes to 6 inch. The pipe serves only the furnace, no other appliances connect into it. I'm guessing when the old furnace was replaced they just kept the original 6 in. pipe in place. Am I wrong to think that I could change the entire vent system to 4 inch pipe with 4inch oval going thru that section of wall?

Maybe....see if you can find the manual for the furnace and see what they require.