I think the Upshur Twin, which I built, is similar. I ended up adding long studs to tie the cylinder heads to the frame. In the first iteration, the cylinder heads were tied to the cooling fins with screws, the cooling fins were interference-fit to the cylinders, and the cylinders were tied to the frame with short studs through a flange at the base (not soldered). I found that the cooling fins started to slide off the cylinders because the interference fit could not withstand the temperature swing and combustion forces pushing on the underside of the cylinder head. With the long stud arrangement, the cylinders are compressed between the head and frame, and the cooling fins see no appreciable axial load other than from the head gasket squeeze. In addition, the engines (plural, because I built six) are much easier to assemble, because the nuts on the prior short-stud flange attachment to the frame were hard to access.