You might want to try this. See if Rootlieb still has the brass saddle-shaped speedster fuel tank filler with cap. If not, you can get a '29 Model A radiator neck and cap from Mikes or other A suppliers to install in what will become the top of the oval "T" tank. You can modify the cap into any configuration you can imagine. The oval "T" tank can be used on edge if you permanently seal the filler fitting and cap and seal off the fuel line threaded opening. This will hide pretty well when facing forward.
I have built a lot of speedsters, mostly A-based. Have a look at my website for ideas:
www.olympicspeedsters.com.