Early blocks are cast just as thick in the crucial areas as Mexican blocks. I've had a '72 mexican block and a '79 302 block side by side. They weighed exactly the same (10lbs more then a 5.0 block) and they both have more material between the main bearings and cam bearings then a 5.0 block. Plus blind bolt holes for the cylinderheads. The mexicans do have the heavy duty main bearing caps, like the 289 HiPo.

As for roller blocks being easier for roller cams, what so difficult about dropping a set of linked lifters in your early block?