[peterpjb]

My question is:

are there any reasons to put NOT a Ford 289 into a small block Cobra?

It is one of the most produced, most reliable, winningest and most affordable high performance engines ever.
It has history, prestige and is the ONLY original engine for a small block Cobra (apart from a 260).
It is pretty available at every corner (at least the sixbolt ones) and you can install some real originality into your car.
It was and is used in many racecars as Cobra, GT40, TVR Griffith, Mustang and many others and destroys six figures Ferrari V12 Colombo engines whenever they meet until today.