I do believe if you didn't need them Rick would of left them off.. When fitting them on my truck the bellows helped when I needed the up pipe to come down to the manifold for a perfect fit, without that little flex they would had never gone on.. You are never going to be able to make the up pipes fit perfectly and with the bellow it gives you a little play.. Just my 2 cents..
You can't build them without bellows or some way for them to expand/contract without cracking or shearing the bolts. No point trying to reinvent the wheel. I think it's cool if you have the ability to build them but do yourself a favor and learn from what already works. Bellows are by far the least of my worries as far as an exhaust restriction goes. Good luck.
Bellows are necessary... And, they're actually not corrugated on the inside, they're smooth... See rick's site https://profabperformance.com/jm/ for cutaway pictures.
You need them because everything expands at a different rate.... Cast block, aluminum heads, cast manifolds, stainless steel tubing (up-pipes). Without bellows, your up pipes and/or the flanges will crack.