Routing exhaust through cross members?

[SonomaZ]

I hope I'm not breaking a cardinal rule by bringing up a 2+ y/o thread...



I have something of an answer, if not for the OP, then whoever may get here by searching as I did.

I'll be using these for my build: https://www.cachassisworks.com/p-27-...-x-2-long.aspx
Seemed an expedient way to go versus hunting for the proper size and wall section in pipe and then having to slice and dice. 4" ID X .250 wall mild steel. I'll be welding these to a 2" X 3" X .120 fabricated transmission crossmember.

So the question is: How to go about cutting a nice matching radius into my 2X3's so I can (hopefully) weld them cleanly and not have it look like I hacked them out with a stone axe?
Any and all suggestions welcome.

Thanks.