Hi. To start, the block may be orriginal to the 68 you're looking at, but, from my info, the heads aren't. 68-69 came with the #5 heads, as you mentioned. #7 heads were 71, 7A were 72. In my 80, I have the engine from a 70 w30.
The block number is the same as yours. 395558 2. This is a basic casting # for 68-70 350"s. I have the #6 heads with the larger 2.0 intakes, which is correct for a 70 w30. I can't help much with the ground off part you mentioned, but others here probably can. If you check out the following link, it will help you definately id the heads. It could still be a numbers matching block, but the heads have been changed. Here's the link, and best of luck. Jim.
http://www.oldspower.com/vb/showthread.php?t=11921