[steve25]

Everything hinges on how much power and or how high a rpm the motor will need to spin depending on your Cam pick.

Your Cam pick will be a result of the average type of driving that will be done with the car and the overall gearing and compression the motor will need for that Cam to make the needed cylinder pressure to go along with the fuel octane you will / can run.

Block wise if your looking for the ultimate strength and have a big enough wallet then go with the 73 block and put on splayed main caps.

In terms of the 70 block if your only looking to make up to 550 hp then the stock 2 bolt main caps fitted with studs is fine, especially if you take the added time to prep the main caps by sanding roll over all the sharp edges but for the bearing parting line.

The 2 boltsmain caps of the short stroke motors are stronger then the 2 bolt caps from the 3.250 inch main motors because they are thicker since they are only bored to 3”, not 3.250”

Some things to note here are these.

1)
On a few 1970 400 blocks I have had that where drilled for 4 bolt mains I have found cracks in the main web parallel to the the outer wall of the outer main bolt holes, so when you look at that 400 block look very carefully at the walls of the bolt holes.

2)

If you build the 400 with just its stock main caps put Allen head 1/2” course bolts in the 6 unused outer holes and red Loctite them in .

This will strengthen the main web area, but also make sure that the Allen bolts you use do not bottom out.

After doing this the line bore needs to be checked just as it would from just switching over to studs from the stock bolts.

3)

The 73 block has a tad thicker cylinder walls then the 70, that’s one of the reasons for the changed part number as they knew catalytic converters where coming and the motors would run hotter.

Personally from the way it sounds like you will use the motor I think either block will serve you just fine with two caps when fitted with studs for just that added measure of safety.