I noticed the tall seat mount pedestals in the floor of the Bellett. I hadn't really gotten that far into the worrying phase.
I looked at a Recaro racing bucket several years ago. Recaro had/has a relationship with Isuzu, and it seemed like a good choice. I tried to sit in it, and my behind would not fit between the sides to rest against the seat surface. Designed for skinny little Italian men. Just like the Italian sports car seating position: Pedals close to the seat, knees against the chest or under the arm pits, arms fully outstretched to reach the steering wheel.
I looked at Momo, and most of their stuff was designed for the same skinny little Italian men, but they had a seat model called "Nascar". I found one of those with more generous width. But I don't think a Nascar fan could fit into one of those.
The description of this Japanese retro racing seat says it is made to fit cars with high seat mounts:
The KAMEARI GT Seat is a great way to combine form and function. It was designed specifically for use in early model Japanese cars, where most modern performance seats are either too wide, or once installed, place the driver very high in the interior. The KAMEARI GT Seat places the driver in a more optimal position, and is designed for the narrow seat mounting locations common in classic Japanese cars.
The RB Gemini has some annoying bracing under the seats which spans part of the way across the car at the height of the center hump. I made a regular style mount with the rails between the seat and the bracket, and my helmet was firmly against the ceiling of the car. I finally redid the mount, and moved the sliders to the side of the seat. I couldn't have picked up much more than an inch of height reduction, but it made a huge difference and now I have enough space to get my hand between the top of my helmet and the ceiling.
The thing that worries me about the retro Japanese seat is if it was designed for a 1960's Japanese man or a modern sized human being (and some of the younger people in Japan are taller than me). If it's made to fit a WWII Japanese infantryman, 5 foot 3 inches and 110-120 pounds, it'll be a problem.