I am going to be running the Ford inline 6 300ci engine. I also will be running a set of EFI exhaust manifolds. I did a little internet research, which is really someone else who tried something, and I bought the Walker Y pipe (41566). It turns out that this fits perfectly.
To get the pipe in correctly (without hammering the daylights out of it) I took the motor mount nuts off. I was able to lift the engine up about 2 inches so that the pipe could be tweaked in and out. I also took off the Offenhauser intake. It gave more room when manhandling the long pipe in from the top.
This was done 11/25/2016
I then took the muffler I bought from Summit Racing. Walker 17827. This is an oval muffler and it turns out it's not the one I wanted. So it's in the garage and on Craigslist now... I went to O'Rielly's and had them order a round muffler for me. I ordered a Walker 17842. This has a 2.5in inlet and a 2.25in outlet. The pre-bent tailpipe Walker 45454 fits over the rear axle well. The hangers are in the wrong places, but I will cut those off shortly. I rented a pipe spreader from O'Rielly's to expand the spots where the pipes join and ended up using some $2 exhaust clamps from O'Rielly's as well for connecting. Welding the pipes could be in the future, but since I am not close enough to starting the engine, we will see if that muffler is going to work. If I need to change it, it will be easier without the welds right now.