If the trans was recently rebuilt, I would suspect something got damaged on assembly. Most likely a seal. Another possibility is that too much end play was built into the clutch pack. The reverse clutch is very big in diameter and if there is a lot of play it will take a lot of fluid to fill.
There is a kit made for changing shift characteristics. It makes no specific VB
repair for delayed rev but does show a VB
plate hole to enlarge for delayed engagement in particular. Email me if you wish to go that route.