oval stack pipe fitting

[canaanmaple]

I picked up an arch with an oval flue collar on it, approximately 9" x 3.5". I am installing 8" double wall pipe for the stack. Does anyone see a problem with using an oval to round adapter that is a few inches wider on the oval end that would sit over the collar on the arch? It would be 9-3/4" x 5-1/4"
(arch came with 7" single wall)

thanks!