Recheck the cam and TDC timing marks. Make sure the cam gear is not moving due to a weak or backed out cam bolt in the front. The bolt does work loose. Do not retorque this bolt. It was designed to stretch and the new bolt from Pelican is only $3.50 and now has the 12 point head.
The belt being so tight sounds like it is almost "preloaded" on the engine between the cam and crank and cannot be relaxed to adjust a proper tension. Are you using a Contitech belt?
There has been discussion about other brands and the way they fit.
Example:
Flennor belts? look kinda cheap, any experience with them
GL
John_AZ