My '65 121 with just the standard downdraught Zenith and M40 has the same boom at 55-60mph. It's a pretty narrow band when you look at the rev counter. One thing worth checking " how's your exhaust hanging dude?" If it's stock they like to be pretty loose on the rubber mounts. Many people think they are too loose and fit an extra hanger at the back to stiffen it up. I think this changes the resonance frequency and makes for more noise through the body. If your cars dry, a layer of sound proofing felt makes the world of difference. The wagon has extra problems with the open back end but putting some felt under the back seat squab and some thick rubber sheet and a layer of carpet in the load area will help. The rubber is always there for the dirty jobs!
Now be quiet!

Derek