From what I can tell, it worked the same way as today's did, based on market valuation.
As a reminder, Eisenhower's America's taxed estates up to 77% and incomes up to 92% (federal income taxes alone!) Of course, back then fewer Americans were pathetic little forelock-tuggers, who spent their days shrieking "socialism" as the prospect of aggressive taxation of the very rich.
Didn't click on it, did you, dummy? It's not someone's blog. It's the Internal Revenue Code of 1954, as amended to August 12, 1957. I gave you the page number. It's a PDF of the original document, copyright 1957, Library of Congress Catalog Card No: 41-26375.
No. You don't need to go to a big bank. We're talking about a loan that would be backed by 60% equity. You'd have a long list of lenders willing to make such a loan, including small banks and S&L's.