The Torso->Leg fittings
IIRC are different between the two, but the difference can be easily overcome with minimal skill and supplies (if you even really need to at all). Like I said there's a couple of legs in the Nob box that are in enough of a "straddling" position that they actually fit on the bikes with little-to-no filing and trimming, so that will help.
The arms fit well enough that most people won't even notice, only challange came from the Kombi weapons as
GW did the old "boths hands modeled onto the weapon" thing that makes conversions frelling harder then they should be. The trick is when making the bikers, the ones that really need the beefier arms (like the power klaws) use the actual Nob torsos first, and way unless you're making a ton of Nob bikers you'll have all the beefiest arms on all the beefiest torsos before you need to stark mucking around with smaller parts. We're lucky that Ork anatomy is so oddball that minor differences in limb size aren't that noticable.
Finally the Nob box has lots of other odds and ends that can be added to the normal bikers to make them more Nobish. Big shoulder pads, various little weapons and wotz-its, not to mention plenty of big angry heads with fancy gobs and such. With some mixing and matching I was able to make 10 obvioulsy Nob bikers out of 3 biker boxes, 1 Nobz box, and a old edition biker I found online (the one with the Kaiser helmet!

) Though I confess that the ridiculous number of power klaws in that unit required that I scratch build one klaw onto a normal sized arms... so one Biker has more of a wrist mounted power can opener, but hey, it's da orks!
Big trick was since this was all for my lady-friend's army, I then had to organize all the weapons, bikes, riders, and wargear in her list and put them each in individual ziplock baggies so she wouldn't get em mixed up when she got around to painting and assembling the little jerks.