As I look at pics of the el cheapo aluminum radiators they appear to have tanks that are taller than the core which means that they would sit in the saddles like the factory. Neither my Champion or ECP are like that.
The summit aluminum rad I stuck in my c10 a couple months ago had the same core/tank height ratio as the original copper/brass that came out of it.
Really I think it depends on what you buy, and from who. It all comes back to measure measure measure. If a rad seller/distributor won't tell you/show you core and tank dimensions shop elsewhere.
As an afterthought, the new OEM radiators like in my mustang don't use the saddle mount system, they bolt in and tanks/core are flush with each other by design.
I think some of the rad makers (not saying names) are too lazy to make them both styles of construction, and just build everything the modern way regardless of how the thing it's going into was originally made.