Well, yes it is worth doing. If your budget does not allow an aftermarket block then that answers its own dilemma. It sounds like you have put together a nice plan of attack and if everything is machined and assembled correctly you should have a fine running bracket engine. I think that the 400 has gotten a bad wrap over the years for notorious overheating. Every 400 I have ever been involved with has had some kind of supporting problem that actually caused it to overheat. They do get thin between cylinders so it would be nice to have the block sonic checked for thickness and crack checked. If it passes inspection I say go for it! Hope this helps.
Hope this helps.
You can also email Jay Roeder at firstname.lastname@example.org