The 509 is a good engine if it's a virgin engine.
As for the French...
2 bolt main is the run of the mill setup for those years, 2 bolts per main cap on the crankshaft.
And siamese are not the pistons but the exhaust ports. Meaning they are touching which is not great because they get hotter in that area.
As for the steam channels, the 400 is physically the same size as a 350 but with a bigger bore which reduces the space left for cooling passages around the cylinders, some end up just steam running through.