I paid about $7K for a 66 426 that had been windowed at some point in its life and repaired. This was for basically a long block. Remember, anything can be repaired but how much do you want to spend is the question. Since this is a 73 casting, there isn't any real historical providence (meaning the block didn't come out of a car) so I would see what a new 426 block runs and compare the condition of 73 against that. In my experience, from a builder's standpoint, it is the heads that generally bring the big money unless the block has some historical significance or value. If it were a 69 or 70 block and you were building a car of that year, then it would have more value just because of the dates (to some people). Since this isn't the case here, it is essentially a RB block and while it might be worth more than say a 440 block, unless it has the rest of the stuff, it isn't worth a lot more IMO. A quick Google search shows a new 426 block going for like $3K.