Just so you know even if you by an aftermarket block, it does not come build ready, we have 2 after market blocks both bowtie and both of them still needed machine work, so if you wanna spend 800 plus depending on what block you buy. Go ahead it is a safer bet, Now we have a 406 with Pontiac heads we race it every weekend this is an old 400 block (not aftermarket), we run 5.80 in it at the track, we also have a 406 with brodix heads it runs 6.90 it is not filled and it has lasted for years with out any issues. Like someone said earlier, it will depend on how you drive it. care for it, 400 are know for there trouble with the steam holes. you can always buy some water weter and put it in the cooling system, have a excellent flowing radiator, a really good flowing waterpump, make sure the entire cooling system is in excellent shape, this will help keep the 400 from over heating and causing you problems.

Personally I would not be afraid to build a factory 400 block,

 

Yes, the SHP block is a precaution, an expensive one. I figure ~$2k for the block and machine work. I struggled with it due to the cost but in the end this is my "big build" and I don't want reliability issues if possible.

The exhaust is a weak link for my build and 3rd genners in general. I am taking it in stages with the intake tract and the exhaust. Even if I fall a bit short of my goal it should net a fun driver.

 

I many guys spend good money on 400 OEM blocks which turn into junk with cracked decks, cracked cylinders cracked webbing and broken main caps.

I wish I could base all my info on just one 400 OEM block LOL.

I have seen 400 blocks crack when bolting a torque plate or head on after the machined work is done make the block junk. That why I get paid 100% up front when working with those blocks.

Never seen a 400 block come all machined either.

Go with an SHP or a SHP Block and don't look back.

 

Yep, going with SHP block. The added expence put me at the top of my budget but "saving" money with the production block and then doing it over would have made it impossible. Cheaper to do it right- once!