I don't think that would be practical or economical. You can buy a used 810G complete for much less than you could probably buy the parts. Additionally, although the lens boards are the same size, I'm not sure about the front of the 8x10 bellows, or the amount of rise afforded by the 4x5 front standard. The 8x10 front standard appears to sit higher, so as to center the lensboard on the larger rear standard.
Also, be aware that the 8x10 G is quite heavy. Nice for the studio but not particularly suitable for field use. I went with a Tachihara 8x10 double extension, and made an adapter for my Toyo 110mm lens boards, along with an adapter frame for the back so I could use the Toyo 8x10-4x5 reducing back.