JCT;
If you can take a look in the middle of the side of the block where a third frost block would usually be, the 400 engine had a raised area but no frost plug. Also in this same area you should see three numbers. If you can tell me those three numbers, I will probably be able to tell you if it is a four bolt main or not. Another way is to look on top of the back left of the block and get the last three numbers from there.

As for using the stock crank and rods, I wouldn’t. They are 30 plus years old and metal fatigue comes into play at some point. Right now you can buy rotating assemblies really cheap and they will have the longer rod, 5.7 inches which is more desirable then the stock length.