I bought a Kilmartin frame through British Car Specialists, as well as many chassis parts for my Mark II, BN7. I was extremely impressed with how identical it is to the original, and how well the chassis parts fit together. I had the most trouble with the rear inner fenders, but Kilmartin does not make those, so they came from England. You can buy the frame parts individually, but it was well worth the additional $1900 to have the basic frame professionally welded together dimensionally correct. From there, it was then possible to assemble and weld together the rest of the chassis without requiring special fixtures (of course it required many assembly trials of the body components to get final alignment).
One down side of Kilmartin is that it may be a long wait. It was about 4 months from order until they built the frame, and then it took another 6 weeks to be shipped from Australia.
Good Luck,
Paul