Stock heat riser measurement question

[92470]

I am wanting to re-install a heat riser on my 71' having a 350 with stock ramhorns. When I had the exhaust replaced, I told them to configure everything as it was stock, so I'm assuming the diameter of the exhaust pipe is as it's supposed to be measuring 2-1/4" at the tailpipe. I have found NOS heat risers listed as 2" and 2-1/2". Can anyone tell me which one I need, or was original? It's tough to measure at the manifold! Thanks