New heat in the shop ..electric furnace help

Here's one chart

usawire-cable.com/pdfs/nec%20ampacities.pdf

If you have no8, and it's UF cable (romex) it's not large enough for a 50A breaker. That is, right on the borderline

As you can see, "it's complicated"

https://forums.mikeholt.com/showthread.php?t=88185

I would be very tempted to toss a 50A breaker in there and KEEP the 40A handy.