I currently run Doug's D-450 on my 63 Valiant and love them. The install is tight, but just be patient and you'll be fine. I installed mine with the engine & tranny in place and found that pulling the torsion bars made it a piece of cake. Top starter bolt is fun, but was regardless of the headers.
Used to have 340 hi-po manifolds, then got Spitfires back in '02 before Harold became a crook. Stepped up to Doug's in '06 and they're holding up great.
The TTI's are a little bigger tube than the Doug's, but last I knew they required a special starter whereas the Doug's fits with the standard Chrysler mini-starter. Tube size hasn't been a performance issue for me, but I'm only running in the 11.70's thru full 2-1/2" exhaust. I think the Doug's might be pushing it to run in the 10's in street trim.
FYI, I still have the Spitfires if someone is looking for a thermal-coated set.