The decision by Triumph to design their own V8 was subsequently proved unwise when they lacked the time and funding to develop it properly. At the time the Stag was in design Triumph and Rover were separate companies and Company rivalry made the adoption of the proven Rover 3.5 L V8 impossible. Ironic since so many have been subsequently converted to the larger unit.
One option Triumph considered in the early design stages was the adoption of the Daimler V8 in either 2.5 L or 4.5 L forms. This was a relatively simple and very reliable push rod 'Hemi' with exceptionally smooth power delivery.
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Triumph however were intent on proving their own technical expertise and had visions of developing a modular family of engines based on the slant four SOHC layout and the smaller Daimler unit, although as powerful as than the proposed Stag V8, was both heavier and wider.
There are Stags running around today with Rover V8's ,Ford Essex 3.0 L V6's, LS1 sbc's and Lexus 4.0 L quad-cam V8's and others in place of their original motors. I'm not aware of a Stag with the Daimler engine but as the owner of a car with such an engine I'd expect it to be a sublime drive.