More efficient propane furnace

I agree about the slim insulation. A few years back I did secure below the entire underbelly one inch solid sheet urethane foam sealed in place by polyurethane spray foam. So the floor ducting is now much better than factory, but I fully realize it’s not up to residential specs. There’s not a lot more that I can do. I have installed full acrylic storm windows throughout that help some with temps but mostly to help with sweating in the cold. It just seems that the current furnaces ought to have progressed in efficiency but I don’t think they have. These old Alpenlites are generally considered to be well made, but again, I do realize they aren’t super insulated. It does have, relatively speaking, okay roof foam insulation.