Do you want to keep it road legal? If not, I favour option 2 (e) Ford smallblock (302) surprisingly compact and eleventy times more tuning potential than Rover V8. Obtain engine and gearbag from vomit spec mustang wreck from ‘Murica. Probably need beefier diff though.
However, suspension and braking upgrades will punish your wallet as much or more than engine shenanigans.
In fact, fit supercharger and be happy.
Merry Christmas, yours uselessly
Sunbeam