I just modified my 7.4 with a complete crane setup as well. I chose the hydraulic cam that they made for the older 420 hp mercs. all is well with the engine and it turns 5300 without any hicups. I can get the part # for you if need be, but its listed on thier website as the cam for 350 365 400 and 420 hp engines. works great sounds great in 7.4's. I modified a 98 gen 5. i wouldnt see a prob with an older one.

Bo - Are you sure the cam is in right? I run a LOT more lift and duration Crane cam (235/245 - .553/.573), have domed pistons (about 0.15" dome), and still have plenty of clearance. I'd check your cam card for where the lobe centerline is supposed to be and put a degree wheel on the crank referenced to TDC and make sure the cam isn't somehow out of time.

-Greg