its a long shot but here go's. how is the clutch line? I have had the clutch line swell when you pump the clutch but i dought it would only do it from 1st to second. I had a 78 truck 5 speed on a l18 that reved to 8500 and you couldn't shift from 1st to second over 7000 . I swapped in a zx na trans and had the same problem . I always just figured that L series 5 speeds could not shift between 1st and 2nd over 8 grand .