[Will]

The date code will be cast into the back of the block on the shelf near where the distributor sticks up. That and the block casting number, located on the bellhousing flange at the rear of the block underneath the passenger side head will positively identify it.

If I was in your shoes, I think I'd use the 350 heads on the 400. When you have them rebuilt, have the larger 2.11/1.77" valves installed and have the pressed rocker studs pulled and the holes tapped for 1/2-13 threads. ARP makes a stud with the 1/2-13 base thread and the correct 7/16" top thread. This will bump your compression ratio up into the high 8:1 range, to maybe right about 9:1 depending on the actual chamber volume of the heads. Perfect for pump gas and better than the stock 8:1 of the 4X heads.

As to weak links - the connecting rod and it's weak bolts are the weak links in the Pontiac engine. No matter what you do, be sure to replace the rod bolts and have the rods resized and magnafluxed. Better yet would be one of the sets of budget forged rods available for under $300 from some of the Pontiac vendors. This will ensure your engine doesn't throw a rod and wipeout all the money and time you invest in it. It also allows room for further performance enhancements if you want to go that route further down the road with no worries about breaking a stock rod.

And, as the others said, everything from the 350 will swap right onto the 400 except pistons. The crank *will* swap and may even be the same casting number, it's just balanced differently for the smaller 350 pistons so you'd have to have it rebalanced if you wanted to use it.