it depends a lot on condition which isn't explained in your post. is it a running engine? is it a bare block? not too many parts interchange with the later flatty. i bought a good rebuilt 60 years ago for 1200.00 and it had about 1,000 miles on a fresh rebuild and was mounted on a homemade stand with gauges, etc.. i also seen one go for 5,000 completely rebuilt with goodies. if it is complete and the block has been fluxed and inspected i would pay 1200 for it. if it is built and running maybe 2500. thats just my opinion, others will have different ideas. if it is just a bare block, tested and inspected i wouldn't pay more than 600 myself. maybe if you provide more info we can narrow it down better.