Which flathead are you working on? Most of the Speedway stuff is pretty good, Offy stuff has been around forever, all of Pat's parts (Wilcap) are well done.
I used an Offy from Speedway before they started making their own. Had no issues with it. Used on a 59 AB with the stock 59AB clutch fork/bearing.
Our adaptor pkg. is the most complete on the market and had been proven over the last 15 years. www.cornhuskerrodandcustom.net Thanks. Gary at Cornhusker
I'd completely forgotten about the starter issue! That's right. I used a die grinder with a carbide tip to clearance my adapter for starter clearance. JB welded the "tin can" shield back in place afterward...
I want to say I had to do the same on an old original Offy...but it's been ages ago. To answer the original question - any of them will work. Depending on your particular combo of parts, one may be easier than the other...
we used one from cornhuskers , works wonderful, comes with everything but the rear trans mount , bolt on everything fit ! jonnybe