1974 360

* OEM 1974 heads tend to be 3671587.
* 3418915 heads tend to have letters by the spark plug holes and originally have 2.02" or 1.88" intakes, including T/A heads.
* 2531894 were all X heads with 2.02" intakes.
* At some point the casting number was moved from the top of the intake runners (under the valve covers) to the underside of the mouth of the intake ports. i.e.: 4448308. - ('80's?)
*Later heads also started getting the larger pushrod holes. Not sure exactly when that started - ('80's?) May be associated with the transition to OEM roller cam & lifters.

As others mentioned, 1.88" is plenty for street/strip. But it's the age, number of valve jobs and the quality of the valve job that are the biggest factors in performance. A 1.88 can outflow a 2.02. Multiple valve jobs "sink" the valve & hurt flow. Cutting a 1.88 head for 2.02 valves can help air flow because the seat ends up fresh & high in the chamber. "Quality" of the valve job & angles depends on the machinist. Aftermarket small block stainless intake valves have more of a "nail" head and tend to flow better than OEM intakes which have more of a "tulip" shape.

Lots of variables in play.