Bill,
I don't know if this will do you any good...and for that matter I don't even know if the 61 motor mount is the same as the 60 engine which I'm running in my 54 truck but I've included a picture from the shop manual that shows the single front mount and explains how to set it up properly to avoid vibrations.
http://rides.webshots.com/album/557190450brdpqh?start=132(Note: to enlarge the page (12) in my "tips and tricks" album, click on "Full size" at the upper right corner of the page.
If your motor plate is drilled with 2 holes....the mount assembly consists of a rectangular steel box (about the dimensions you gave) a rubber insert (not poly that I've been able to locate), a top 1/8" steel plate that sits on top of the rubber insert and a thin dust cover that covers the whole assembled mount. The dust covers and rubber insert are available from most vendors.
The rubber insert can be inserted upside down if you aren't careful and the seemingly insignificant "clearance" they recommend is not insignificant. The replacement mounts are just a tad thinner than the originals and to check the clearance you should clamp the mount in a vise to simulate engine weight. If you get in a hurry you will be reminded of your haste each and every time you let out the clutch. A few quick passes with an angle grind (as recommended) will establish the correct clearance.
I wish I could help you locate the elusive "box and top plate" but I know of no vendor that offers those 2 components. Good luck with your search.
Dave
