Basic difference is the engines are totally different.
The 2.8 is a german "cologne" unit, where as the 3.0 is a "essex" unit.
The essex is getting thin on the ground, but would be easily the prefered engine due to it not having siamese ports, like the 2.8 does, although ARIZONA HEADS can be fitted to the 2.8 if you can get them, these are NOT siamese port and will have the potential for a lot more flow.
If I was dragging a capri I would look at a small block ford or chevy engine, parts are dirt cheap, easily obtainable, VERY tuneable.
The chev in particular is the most prolific engine in the world for example.
If you had the budget(?) 1000bhp is childs play, 2000bhp is possible depending as I say on budget.
tabetha