Could he use the 1.90 valve to clear stock pistons? Do AFR do that size as alternative or only 2.02?
It's just that he sounds more comfortable in not having to do machining or order custom pistons (although if he's rebuilding engine then he may as well) and like UK he probably has access to higher octane pump fuel than in US, so could even take advantage on that if he wanted to go custom piston route.
On our Mustang 289 race heads we run 1.90 inlet and 1.60 exhaust, we were told by race company in US that there is no point in us going to 2.02 intake as the iron heads and crappy Tri Y manifolds we have to run can't take advantage and heads won't flow enough (same comment as you've made about iron heads). We're also mandated to run to max 7,000 rpm through sealed limiter on MSD box.
May be different for Ali heads which agree are far superior but I wonder if this might make life easier for him??