I'd first call Brownells and talk to a tech.
That failing, I'd find a friendly gunshop with a large stock of Weaver bases and start trying various ones. Level the bore first and try bases till you get two that will also be level.
Given the rifle's vintage I think it may have had a target type scope on blocks (Lyman or Winchester) as even a Lyman Alaskan has a maximum mounting length of about 8.5"
Now, since it not really a collectors item, the easy way out might be just to get a one piece Weaver base that fits and punch more holes in the receiver top,