According to the research I did, the widest tyre you should fit on a 4.5" rim is a 175. I've gone for 175/70/14's on my MGB wires, while Phil J has fitted 175/80/14's to both the cars he's built. I think they look great and they don't stiffen up the steering, but you can fit 15" MGF alloys relatively easily (same PCD and bolt pattern as the Triumph, but I believe you need rear spacers) which allow much wider tyres if that's what you desire.
If you haven't already seen this comparison calculator it's a very useful tool for checking the height and rolling diameter of tyres you're thinking of fitting -
http://www.kouki.co.uk/utilities/vis...ize-calculator
As a rule of thumb, 175/80/14 are the tallest tyres I've seen successfully fitted to a Sammio, so if what you're considering is any taller they'll probably foul something.
