... Hella Comet 200s are about the biggest you can fit without them looking completely silly. Rallye 1000s seem huge.
I had mine attached to the bonnet as that was the "easy" place; they looked OK, but the attachment was a pig and it meant moving cables around every time I removed the lid.
Make sure you have the ref 37.5 versions, but then you MUST remove the normal high beams as otherwise you will get into trouble, if, of course, you want them to work as the originals. If you're prepared to use another switch, then that's also OK, but the max allowable ref is 75 from all light sources (and the number is per lamp, not per pair).
Bret