Yes of course it's all hindsight now but it must have been hard trying to compete with the Rover under the same ownership. I'm a bit rusty on the dates but IIRC the Stag was based on the 2000 MKI that was replaced by the MKII in the early 1970's using the Stag design even though the Stag hadn't been released yet so they wouldn't have been owned by BL at the time. Saab were using the Dolomite slant 4 in the 99 but decided to turbocharge the 4 rather than do a V8 conversion.
My daily driver is a 1977 Triumph 2500 TC Auto that I hope to convert to manual + electric overdrive when I get the time. It is a fantastic car and like the Datsun of the same era can handle all the corrugated roads that a newer car wouldn't cope with. The trade off is a less precise road handling experience though. It returns 24 MPG from the straight six without any of the issues the Stag had.