I picked up a 400 crank for a possible 383 build but it has had some modifications done to it that I don't recognize.
It has some numbers stamped on the rear throw weight and what looks like some weight added to the same throw.

Anyone know if this means it was internally balanced at one point?
Can anyone decipher any of these numbers?

 

Doesn't matter, you're going to have to spin it up to balance it again anyway.

Why start with a 400 crank to build a 383 these days?

 

Anyone can buy an aftermarket kit. I have friends that have machine shops and swapping and trading parts for machine time is what I like better rather than spending cash.

 

Have you measured the rod and main journals?

 

Yes, They are all 10 under. The mains have not been cut to fit a 350 block (yet).

 

Aftermarket cast cranks are stronger than factory cast cranks from what I understand.

 

Yes, they can be stronger. A forged crank would be much stronger than the stock nodular iron crank I have pictured in this thread. Even a heat treated nodular crank would be stronger. I might buy either of those for a high horsepower build but this one will be for the street.
Does anyone possibly know what the numbers might translate to?

 

I don't recognize the number stamping but the Mallory weight welded in is for balance weight required when it was balanced the last time. Internal/external doesn't matter when the rotating assembly is balanced.

If it's a factory 400 crank, you need to provide a 400 SBC dampener (preferable new or aftermarket) and a 400 SBC flexplate/flywheel when you take it in to be rebalanced to match your rods, pistons etc. If you're not willing to get your rotating assembly rebalanced, sell off the crank to someone else. It will be cheaper in the long run.

When a rotating assembly is balanced, adding or removing weight depends on the weight of the rods, pistons, wrist pins, rings, rod bearings, rod bolts. I've had a couple of cranks that needed weight added and my last few had weight removed.