Sounds like a good deal $$.
the obvious? you could always go with 906 heads? with 86 cc 11 to 1 CR page here
I have been thinking of building the same combo, a set of the 251 are on ebay now but to much for used IMO. I have a 400 ready to go. I priced the crank work and was quoted $200.00 to turn the mains down to the 400 journal size and grind or polish the rod journals. I was also told the bob weight {or weights} will need to be turned for clearance which may add to the balance cost.