Plumbing Vent Problem

[Thalweg]

When I bought my 1962 Tradewind, the all the bathroom fixtures were missing. I’ve recreated everything as close as I could to what it was originally. I got a new pie shaped black tank from VTS, built a new cabinet, and plumbed the supply lines. Now I’ve got to plumb the vents. I have the original vent stacks in the closet next to the bathroom. It’s shaped like an upside down Y, with one leg to vent the black tank, and the other to vent the drains. The black tank vent is easy. The drain vent is a problem. The pipe has to pass over top of the black tank to reach the vent stack in the closet. I’ve got to build a platform for the toilet to sit on over the black tank. So I figure I could either put the vent pipe below the platform, which would require elevating the platform more, or I could run the vent pipe along the back wall above the platform. The existing vent pipe is 1.25” diameter. My question is; is there any reason that I need to stick with that large of pipe for the vent? Would there be a problem with reducing it down to about ¾” where it passes over the black tank? This would allow me to raise the platform much less than with the 1.25” pipe.

Additionally, if I am able to put the pipe below the platform, I’ve got another problem to contend with. The black tank has a 3” female threaded fitting for a male threaded toilet flange to screw into. I can’t find an adapter to allow me to raise that.

I’m sure I’m not the first one to have these problems. So I’m curious how others have handled them.