So my silvertag is no good. Should I be working on the 1971 its better?
With a few exceptions, an ST is just another K0.
The earliest ST frames (and damned few of them) lacked the crossbar, which connects the RH & LH sides of the frame, just above the engine, forming the bottom of the triangle. Other than that, they're all the same as any other K0 frame with the exception of the hole for the tail light pigtail...which is virtually a non-issue. K0/HK0 frames have been known to develop stress cracks, on either side of the triangle above the upper engine mounts. (K1-later frames have the one-piece internal engine cradle, which eliminated the stress cracking.) Weld and OEM finish quality is wildly variable, throughout the K0 production run. The later K0s tend to be the best. Stress cracks are relatively uncommon, easy enough to weld.
Complete & "correct" STs tend to command a modest premium over other K0 & K1 models. As Pat mentioned, pointedly, the earliest production specimens have the most resale potential. As the ST era progressed, Honda made a lot of small running changes, none of which are well-documented...hence the diminished market premium of the later bikes; there's no way to really know what was factory-correct, other than an unmolested bike in a remarkable state of preservation.
Most of the differences are bolt-on parts, some overlap into the BT era. The toughest items to find are the early-style muffler, wheel hubs, seat pan (in usable condition) and speedometer. The ST speedometers have plastic high-beam indicator jewels, they're just flat plastic; the fabric jewel is urban myth. The silkscreening is different and the dial is only supported on one side, internally, making them susceptible to breakage due to engine vibration.
Nothing really wrong with building a rider, restomod or custom on a K0 frame. If starting from scratch, I'd prefer a K1. If you're ultimately going for a full-on resto, K0 is a safe bet and an ST should be at the high end; it comes down to what you see as being worthwhile. Sourcing a full compliment of ST-specific parts, from scratch would be a challenge and might cost enough to leave you with a net loss, come auction time.
