We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Duncan Wood"
saying something like:

Thermall Capacity of iron's .46 KJ/kg, thermal capacity of aluminium is
.91, but engine blocks tend to be similar volumes rather than weights and
allys a1/3 of the density. But given ally's 3 times as conductive I
suspect the whole engine warms up evenly,

Which is right, ime. Ally heads and blocks distribute heat better and
alleviate hot spots. As which actually warms up quicker overall, I've
known both of equivalent sizes to warm up at pretty much the same rate,
but it's all down to age of the design and the cleverness involved. For
decades now, thin-wall CI blocks have been taken for granted and they
tend to warm up a bit quicker than traditionally thick castings. Overall
though, I'd say ally warms up quicker overall (ie, the heater works more
quickly), but CI warms up the combustion chamber quicker - and that's
just down to the conduction factor.