quote TigerNewbie:

So, I get it. So my idea of improving the 260 block by boring it out is out of the question. But what can be done to make the original Tiger 260 more of a “high performance” engine while keeping the engine externally original, if anything?

My $.02 –
There is little externally to differentiate between the 260/286/302 engines, certainly it would take a [highly] trained eye to spot the difference. When the "updated" engine was installed in our Mk 1a, a lot of the visible 260 parts were re-used. (yes, of course I kept the orig. motor)
Unless you are going for "Corvette concours level" restoration, any black-painted motor will be OK.
Good luck.