Couple ramblings from fruity....
Marsland had the 109 stw frame wrong for a long time, I also have heard rumors of other little errors. They basicly make them in bulk to be the cheapest in terms of price.
Richards, ex Steve Walker chassis used to be the best in terms of originality and quality, but since Steve left(was forced out) or what ever you want to call it things have been down hill. They have stopped making Series 1 frames, and the quality of the SII(a) and III frames has dropped as per account on various BBS's.
I doubt you will see any US makers of frames anytime soon, as it seems steel is at a premium in the US.
Overall I think Marsland is the smartest option in the US, price and quality being the top reasons.
Lastly Series 1 frames it the only difficult one, either finding a solid donor or getting somehow onto the list for a walker frame which is more then tricky.
1967 MGB convertible
1966 SIIa 88 Softop Perkins Prima Powered
1964 SIIa 109 Rosenbauer TLF
1976 1ton Rapier missle Sankey trailer
1996 BMW 1100 GS
wanted ! 107 wagon / 110 wagon v8 or 300tdi