I am struggling with lenses and hope for help identifying lenses that would solve my problem, chiefly a problem of fall-off with lens fully open.

I am shooting 8x10 wet plate portraits (full standing figure). Due to slow effective film speed and my lighting choices I need to shoot with the a lens full open, f6 or faster is better. The lighting and exposure time (10-30 seconds) are necessary to get the look I want so going with strobes and stopping down is not an option. I need even coverage into the corners including with movements (I tend to move the front lens standard up and down, mostly up). And, lastly, I need the camera pretty close to the subject (8' is ideal up to 10' is possible).

I'm thinking that the only way to get fully even coverage with an 8x10 fully open is to use a lens designed for 11x14 or larger in which case I believe I need a pretty wide angle for 11x14 to keep the camera fitting in the room (lens 8' from subject).

I have a few lenses but none are giving me even coverage to the corners. What I have:
Fujinon W f6.3 250mm (covers 4x5 beautifully but looses light in the corners for 8x10)
Ross Express 10" f4.5 (too long for my needs for 4x5, uneven coverage for 8x10 at corners)
Ross Rapid Symmetrical 5x7 (brass barrel lens) approximately f8 I believe. (came with an 1880s full plate camera works great for that)
Ilex Paragon Anastigmat 5 1/2" f4 (popped this one in last week and it looked terrible - uneven focus within a 4x5)

I prefer sharp over soft and I need even focus and even illumination to the edges.

Budget of up to $1500.

Thanks for any advice!
Alexandra