If you dig back through published information, the J-58 is an interesting engine. The original "simple" single-spool turbojet wasn't that much different from the DeHavilland Gyron. It's the adaptations made to fit it for the A-12/YF-12/SR-71 family that made a major change in it. The by-pass ducts from compressor to afterburner are quite impressive in size (once upon a time, over three decades ago, I worked at P&W's Florida facility where that engine, along with the RL10 and F100/F401, was developed; they were still doing overhauls there and they had a full-scale mockup for some uses. I wonder what version of that engine the V-418/419 and F-106X proposals used, with all the bypass or without?