Sometimes i take things for granted, thinking everyone has access to welders,neighbor or somebody. Sorry. I am also a wood worker rebuilt my 200 year old house,cupboards and all. Can't afford to newer at the time,so did it slowly one room at a time.
My next thought if i were to build a frame for Amilcar i would use plywood in layers up to the thickness of the frame. A lot of machine shops have scrap multi layer like 1/2" 5 layer plywood that parts come in and goes to scrap, sorta like baltic birch. Just cross grain glue up pieces, be way stronger then a single cut timber.
Like them pictures,some really cool frame work. Recognize the rear late 40's Ford fenders on the buss. The body frame work is a lot like the old model T's and A's ..........Curt