Old Clark 5-speed overdrive HOWLS!

I have an old Wonderbread truck converted to RV. It's been retro-fitted with a cumins 5.9 12-valve mated to an old clark 5spd OD and an Eaton 2-spd rear axle... after an hour or so on the road, the gearbox warms up and as it warms it starts to howl, growl and whine... it gets so loud I put in my earplugs to avoid fatigue. I know the gearbox is old but it's still shifting smooth and there never shows much particulate after oil changes. But it IS loud- I am told it's the OD (5th) gear that is so small and takes so much load it just wears out the bearings... I have also been told by a Heavy-truck mechanic that the noise is resonating into the van body because the mounts are bolted to the transmission- he said there should be two mounts forward on the engine and two mounts back on the bell-housing and none on the transmission casting/box itself. Can anyone comment on this problem? I'm about to go deaf! Whaaat? Deaf, I say! Thanks, all.

Old Clark 5-speed overdrive HOWLS!