[WADS56]

I would get a good used HO block and throw a 347 stroker kit in there. There is no reason to stay with a 331 if you are not going to spray, or blow it.
If you are going to stay around 400hp there is no need for a expensive forged kit either. You can get a basic stroker kit with I-beam rods, cast crank, and even hypertuctic pistons if you want. Throw in a set of main studs, and a girdle, and it will last a long happy life.

Don't worry about 5-speed blocks having to much end play. If it is true the block has nothing to do with it. The thrust bearing is what determins crankshaft endplay and you will be geting a new one anyway.