I think the stuff about reducing the quality of the steel hardening is a myth. As far as I understand it the stroke is simply too long and the crank too heavy, on later engines, for the 3 bearing setup to cope.
Even without the hardening quality issue, the problem was there from the introduction of the "big crank" 1300 in the 1970> Herald 13/60 (code GK) The big end journal size was increased without attention to lubrication so the same amount of oil had to lube a larger area - and it mostly didn't! Then the 1500 came along with an increased stroke and even more weight which made everything worse still!
This is why, if you want to build a screamer, the 1300 "small crank" Spitfire engine is so highly regarded, it doesn't have the inherent flaws of the 1500 and can produce more horses for longer than any 1500 ever will. However, this motor is now getting rare (and expensive) and we work with what we have, so......
All the 1500 savvy racers say the best improvement is to cross drill the crank to get better lubrication and go from there, uprated pump, oil cooler and very good oil follow that. Lightening and balancing the rotating masses is good for pickup AND longevity, then add your choice of bolt-on goodies. My choice, a decent TR5 spec cam, ported, matched and flowed head, 4 branch and extractor exhaust and a single 45DCOE. Oh and a 123 dizzy or Megajolt. If you're REALLY brave (and richer than me) getEFi too!