Scott,
Even if you don't get any dimensioned drawings from any archives, you could always
take a laser "tape measure" with you to the Museum of Flight and get dimensions. I
don't think it would do any damage, and the place isn't crawling with docents or
security. And as it's easier to ask for forgiveness than permission, I'd just go
for it, if it were me...

If you make it up to Seattle, there's a period Gemini mock-up at the Pacific
Science Center, near the base of the Space Needle. It used to be one of the
central attractions, with a line of kids waiting to climb up and sit in it. I got
my minute or two of seat time back in about 1967. But now it's in a less-visited
corner, with the access steps still in place. Bring some corned beef sandwiches
and waste disposal bags, and you could probably sit in it unnoticed until you
break a Gemini endurance record

Dale