Yes, that was a plan. The toilet would be draining down the stack. As I mentioned, one plumber friend said that would work, one said it wouldn't. Honestly I don't particularly trust either one as pretty much anyone with a truck and tools can call themselves a plumber around here. I can easily run another 3"-4" pipe down for the toilet so that's what I will do to avoid the issue since it would tend to pull the traps . Theoretically, there wouldn't be a problem if I tie that into the stack as long as it's tied in below the venting point for the 1st floor fixtures, right? The main drain to the septic is about 6' up the basement wall so there isn't a ton of room/slope to work with down there to tie it all in. Would work much better if I can tie it in somewhere on the 1st floor.