1. All the bolts are metric on the 8.1. I bought a core 8.1 years ago, the previous owner decided it was a great idea to cram 3/8-16 bolts into the 10mm bellhousing bolt holes when he mounted it to an engine stand.
2. Despite being metric, most of the bolt patterns are the same--so headers will bolt to the heads, but with metric hardware, plus all the usual problems with headers and tall-deck blocks. Motor mounts bolt to the side of the block, etc.
3. The oil pan is wider than the previous big-blocks. Mk IV, Gen 5/6 oil pans WILL NOT fit.
4. Flywheel/flexplate is neutral-balanced on the 8.1.
5. Cylinder heads have three bolt-holes moved over "a little". Older heads will not fit on the 8.1 block without fairly major surgery to move those three holes. And they only move about half the bolt diameter--so it's not like you can just drill and tap a new hole. Coolant passages are different, too. HOWEVER GM built one "show truck" with older heads on the 8.1 block. It can be done...if you have enough money and courage.
6. Firing order is the same as the LS. intake ports are evenly-spaced. Be careful swapping cams.
7. Far as I know, there's only one oil pan for the 8.1; and it's cast aluminum. Perhaps there's more than one design, but I don't think so. And I don't think it clears the crossmember/axle of a GMT400.