I run a t-5 behind a flathead in a 36. It is indeed a great swap. Speedway does indeed sell the Offenhauser adapter as well as their own name brand. As stated above Cornhuskers sells adapters and If I am not mistaken Red's Headers also sells them. I purchased my adapter from Speedway but bought my polished heads, intake, and coated headers from Red's Headers. They are also a very good company to do business with.
The flathead, T-5, and a 3.70 or 3.90 rear end is a really great combo. I am sure you will like it.