Mike, I think I can help you. Were the #s from the front valley pan rail?
MR was the code for a 66 401 in a Skylark Gran Sport. It was called a 400 because of the 400 cubic inch limit in the intermediates, but it was a 401.
290 is the date code. The engine was built on the 290th day of 1966.
6 is for 1966.
K is for the assembly plant code. K was the Kansas City, Missouri code.
117505 is the car # in the series.
The actual casting #s are embossed on the back of the block in #s and Letters that are about 1" long. The #s you gave involve the production code and part of the serial # for the car. Yours tell us much more then the casting # would.
I hope that helps. I wonder if that is the long lost engine that came with my car?
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