Hot Rod Magazine did a test on a number of different manifolds a few years ago. They showed the Slingshot to give better performance than the Thickstun PM-7 / Tattersfield. (Your mileage may vary.)
Wow, a lot of interest in this topic. I found the Hot Rod Magazine article rating the Slingshot well above the Thickstun PM-7 / Tattersfield.
There's a lot of outstanding information on this thread.
Moving very slow on building my ride (1941 Ford Special Business Coupe

trying on a lot of different ideas for size. After reading all of the replies and the HR magazine article, sounds like a Slingshot is the ticket.
Looking at used intakes and Stromberg 97s but also looking at brand new. Money's not the issue. Driveability and cool are both important. Seems like the old manifolds will work just fine but sometimes rebuilding an old Stromberg can be a crap shoot. My performance goal is solid, snappy low end, off-idle, performance from stoplights to cruising speed of 35-45 mph.
Hard to tell if a Slingshot with dual 97s is going to make any difference in the performance of my stock '41 221 cid flatty. So long as it doesn't make it worse, I might go for it just for the cool.
Current intake is stock and current carb is a leaky, stinky self-rebuilt Holley 94 that dribbles gas on my intake. Looks like the gas is coming through the idle adjust screws. Also have an annoying off-idle stumble. Even a single new out-of-the-box Stromberg 97 sounds good while I make up my mind about a 2x2 setup.