If you ever see a diff pin protruding from where it should be, the diff is JUNK period!!
If you have reverse, you know that the you have a solid connection between the 2nd clutch drum and the front planetary. So if you have a good 1st gear and it goes to neutral when it is supposed to upshift into 2nd, the valve body is making the 1-2 upshift. So either 1-your 2nd clutch is not holding, 2-you have a problem with the 2nd clutch hub on your input housing or 3-you have no oil going to the 2nd clutch.
1957 Nash Metropolitan
1960 Bonneville Conv 3x2 1964 Bonneville Vista
1965 Bonneville Conv
1965 2+2 455 3x2 and 1966 421 2+2 3x2 (donor car)
1967 GP Conv