Holy crap!
Why is someone suggestion oil viscosity set by Ford at the OE level?
I can almost guarantee any new ford engine has way tighter bearing clearances than an after market, machined and built engine.
I agree listening to the machinist who worked the block, but even that should be heeded with a grain of salt.
You, individually should mock up the engine and measure each clearance and then have enough information to base a decision on what viscosity in regards to application, load and clearances. How hot the oil average is, is a huge factor.
I said builder, not machinist. I'm assuming someone is rebuilding the engine for him. If HE is rebuilding the engine himself, then yes, he should be basing his decision off the variables you pointed out.