Posted sone pics of the length and shifter location of the RX8 6speed (5 is a tiny bit shorter).
I think I would be going for a cut & weld bell housing to mate to the V8, far cheaper. I found (can't find now ofc) a locost method was to put the gearbox onto a spit so it rotate around input shaft and a fixed output tube over the splines for output. Then a frame to carry the angle grinder and once started clamped down. Just rotate the whole gearbox to get the cutting disc to chop off the RX8 bell housing. Think it was about 3" but it's best to match the best place based on both bell housings diameter as to match the best place to cut.