The main reason I own my SPF is that I don't fit comfortably in a Tiger.
Yes, all the Mark I and IA's came with 260's and the Mark II's came with 289's. There was a long list of LAT options to order but the more popular were the Cobra style valve covers, 4bbl carb with F4B intake, and the LAT 70 wheels. Solid cars are starting to push the mid to high 30's now.
The overheating problems are known and easily corrected. My friend's car which has a VERY stout 289 can idle in stop and go traffic in 100 degree heat all day long.
Three good Sunbeam web sites.
Our local club
http://www.stoa-tigerclub.com/
My friend's side business with cool Tiger stuff.
http://www.tigerengineering.net/
Sunbeam parts galore.
http://www.rootes.com/