I think you would need a serious build to need the flow of the st head but then you have the vvt to sort and will the cams suit the turbo ? The thicker gasket won’t drop the cr as much as with the Blacktop head.

the black top head on the mk1 Rs often does 350~400. There is no vvt to play with, the ports line up to more manifolds and this is more than the st/bt combo above is reported to be good for.
The Rs does have sodium filled exhaust valves.

It’s the combo of the St pistons and rods with the gasket and bt head that make it more robust than just a Blacktop. The rods have a bigger gudgeon pin and bigger big end bolts.

Mines only being built for 260 ish any more and I would just forge a new crate Blacktop.