I have always done mine off the engine. Whole lot easier. With the cams out of the way, a standard valve spring compressor works great which I like best. I'm a little old school. Just remember prior to installing the heads to put your crank timing mark at 6 o'clock and cam timing marks at 12 o'clock. (Once the heads are installed, these marks will be 10 o'clock passenger and 2 o'clock driver due to 45 degree angle.) The valve springs may prohibit you from getting them exact just get close. In this position, all pistons should be below deck and valves closed. If you're reusing the old timing chain, you may not be able to see the brightest links due to wear. Simply lay the chain out and pull the slack out. Mark the link at each end that is 90 degrees and presto, you have timing marks. Ford made the 4.6 very easy to time. Once the chains are installed(minus tensioner), an 18 mm on break over bar on the cams will get you lined up. Don't need any special tools like they say. If your cams are greater than 262 lift, you could have ptv issues with stock pistons. Once it's together, ALWAYS turn the engine without you plugs. It should turn freely. If it doesn't, something was done incorrect. Hope that helps. I've done many 4.6's this way with no issues. Good luck.