I am asking for suggestions to lighten the 26.6 aluminium seat-post and quill stem on my current Neo-Retro project that I've recently finished..The new modern drive train and custom built wheel-set has turned a 1970's frame into a real nice ride...but if I could think of something to replace the seatpost, quill stem and especially handlebars I could really make it Great Again.

Who has good suggestions?

Bike shops...
Kalloy makes 2 lines of seat posts in every diameter, including 26.6mm...

Seems to me that the OP is making this bike into something it never was intended to be, not wrong but not "Great Again" either.

Stem and post weight is a funny thing. Both being fairly high up off the ground contribute to what I'll call "rocking weight", the feel of how easy it is to rock the bike back and forth while standing. Yet as otherwise stationary weight they do little to change efficiency. I'll add that I have seen far more riders fall from broken posts, stems and bars then I've had riders tell me that they only got up that big hill because their post was 58 grams lighter. There's a reason that most stems are solid, as example, and a reason that some of the hollow stems are called "death stems".

What I have done with common parts is to cut off any excess. For a post that's any of it's length after about 3" of seat tube insertion. For bars that's the last bit of the drops, I never go down there anyway. But the stem I leave stock, my helmet lacks a face guard. Andy

I doubt most people could feel reduced weight of a bike by replacing stem and seatpost.

Drill holes in every available surface. Fill the tubes with helium.

If everything is already aluminum, then you're not going to be able to make any crazy improvements. I would only make changes for the sake of fit and ergonomics.

If the seatpost is one with a separate saddle clamp, then I completely understand... I've always hated these, and I'm about to replace one this summer on a bike I keep in the USA, and that post is even aluminum!