Scott,
A few questions here: Actually I am going to go all philosphical on you in my response.
You note the one of these rifles is missing the butt altogether and the other you would like to upgrade. Now, what is the end goal?
The Achiever is definitely an introductory rifle while the 64 is a great intermediate rifle for the shooter starting to get serious about competing, an excellent but still a stepping stone rifle. I point this out as butt hooks on rifles for shooters whose development is not ready for them can be more a hinderance than a help. The typical newer shooter does not have the physical body awareness to feel if the shoulder is tensed compensating for a poorly set hook. They will simply force the shoulder to compensate, slowing or stopping development.
This is where the simple rubber buttplate with a modest amount of vertical adjustment is really a boon. So long as the butt is reasonably adjusted, i.e. up for prone, middle for kneeling and down for standing the shooter in time will more or less automatically find the best place on the shoulder to place the butt.
If you are trying to make the rifles seem less barrel heavy, try some weights in the rear of the stock to move the CG rearwards.
Now, having said all that, (consider it a product disclaimer) Champions Choice has a line of modestly prices butt plate products
http://www.champchoice.com/cat-Butt_Pla ... s-637.aspx where they can likely either help you replace the missing butt pieces or place basic hooks on these rifles without breaking the bank.
Just my 2 cents
'Dude