The nickle content (strength) was gradually lowered over time as HP numbers decreased. Rated at 185 HP, the block is not the reason its rated as such. The reason for HP ratings is mainly found in the heads with the secondary being the cam.
You could take a set of #16 1968 360 HP GTO heads (incidentally only 330 HP in a Firebird) with a cam swap and have a 360 HP block. The reason pre 71 blocks were so desirable was that the net HP ratings were usually (and actually) off by upwards of 25% , thus the increased nickle in the block to offer a "margin of erro.r"
Would a 73 block suffice at 400 plus HP? absolutely.
Additionally, the 500557 block has lighter webbing between the cylinder banks... decreasing its strength. However, it is widely agreed that these blocks will easily support a 500HP street machine.
My 2 cents...
...and if anyone is interested, I have a 500557 block available.
I'm a hobbyist. Not a professional. Don't be hatin'!
