Wanting to run my exhaust under the car instead of the fender exit like it is now and my ground clearance is tight to say the least. Can I run out of the turbo in 3.5 round down to the frame rail and then go to oval tubing under the car? Will this hurt the spool time or cause a restriction?

Mine is ovaled (via hammer and torch) as it passes between the torsion bar and trans. No issues.

I'm wondering how far from the turbo I should go to start the transition? If I could do it in the engine comp. that would be great

I bought the 3 inch oval tubing, can't remember the brand name now but I can go look in the shop for a shipping box if you want the name of the company, for my 1940 Ford street rod and it wouldn't fit into the stock X member holes in the frame so I ended up buying the 2.5 inch. Those pipes are wider in the oval than the round parts They sell the transitions as well as the curves, pretty good quality stuff also

I split 2.5" pipe and added a 2" wide center strip to make my oval pipe.Did it to fit between trans,torsion bar/frame. 3.5" round is 9.62 sq. in.,the oval worked out to 9.908 sq.i.n.It goes oval about 18" after the turbo and back to 3.5" round under the car.Priced some of the stainless nascar stuff and decided to make my own.

All of the NASCAR shop use & sell used oval tubing from their old exhaust systems.
Vibrant, Burns & other also sell it.

Just a comment?
Round and oval are the only shapes used now, but back when Prof. Gordon Blair ran simulations of a square exhaust in his giant QUB computer and it showed a slight increase. The real-world testing didn't do as well, so he stopped the project - but the loss was very small.

Use whatever shape you need to get past the obstacle.