i don't know which Washington your in but Joe and OMM are both in reasonable driving distance even from the Rt 31 goes through.So there's two 340s and all you did was start a discussion. When I wanted a spare 340 I waited and watched the ads and then bought one.
For what came in '69, look for the magazine reviews (some reprinted by Brooklands books, others on the 'net)
Factory sales lit is available at
The 1970 Hamtramck Registry Home Page Go to the Library. Then 1969.
From a '68-72 318/340 doner car I would get the K-frame, the throttle cable and transmission linkage.
If going with the 727 automatic (which is the automatic used behind the 340) you'll need that throttle to transmission 'kickdown' linkage.
(The dual plane intake height calls for some differences from the single planes 2bbl LA engines like the 318 and 273 often came with.)
Your choice on brakes will be drum/drum or disk/drum. Depends how many repeated high speed stops you think you'll be making. Either can be made to work well (but would NOT keep 9" drums if thats what it has now) for everyday normal use. The factory K-H disk hubs are small bolt pattern.
Yes upgrade to the 8.75" axle to be period correct. It has a distinct look and of course is stronger. I'd only go large bolt pattern if you are looking for certain tire and rims. I didn't bother but it would have made some things easier (and cheaper). I run the classic 3.23:1 diff with close to factory diameter tires and get reasonable performance and milage considering the cam. (my autocross tires are a bit shorter).
Factory 340 cam will be torquey and there are a few after market cams that are pretty similar if you want to squeeze a little more omph. Decide if you are staying factory intake and carb.