[br850]

bit late to this one and I may be stating the obvious but the cams run direct in the housing so any bits of grit or dirt tighten the clearances.
Is there an oil mark around the cam or housing, by this I mean the brown line at the edge of the running face and on the cams. If you look at a crank pin there is the shiny bit where the bearings run in the middle but the outer edges may have this line usually on the main bearing housings.
Its only a few microns but it does make a difference.
Scrub everything clean and use some very fine emery to remove any burrs and oilmmarks. Pump lots of oil through the galleries because the oil may be carrying crap out into the housing.
When you have done this wash your hands thoroughly and re assembe. The cams should spin in the housing with minimal pressure.
The only other thing would be to get an engine reconditioner to check the line bore on the assembled housing but if as you say they are a matched pair they should be ok.
Good luck