is the 10w 30 to light for these old motors?
No, 10W30 is not too light...unless that engine was specifically built for racing us and has unique oiling requirements, or if it needs higher viscosity lubricant due to component wear.
The OEM oil recommendations (from the Factory Service Manuals) were as follows:
For operation in temperatures from -30F to +20F: 5W, 5W-20 or 5W-30
For operation in temperatures from 0F to 60F: 10W, 5W-30, 10W-30, 10W-40
For operation in temperatures from 20F to 100F or above: 20W, 10W-30, 10W-40, 20W-40
I also use the Brad Penn 10W-30 that Matthew recommended. I believe the higher zinc content is better suited to the flat tappet cams/lifters. If I had a roller cam & lifters, I would probably use a "normal" 10W-30 brand.
Double check the capacity; I think your 400 should take 6 quarts with a filter change.