Slight speedo cable leak...and other questions

I have not had problems mixing type F with Dexron, but that was a long time ago when Dexron was just type 1. If it's not synthetic I don't see why mixing any Dexton would be a problem, but I'm not a ATF guru. Maybe TB knows more?

When your putting it in reverse and it's delayed engagement is this the first time in the morning when you fire it up to back out of the garage or all the time? Torqueflite's are prone to torque converter drain back when they sit overnight and since they bypass fluid in park it doesn't fill the converter. Without doing that it's sluggish the first time in the morning, or after it's sat for a few hrs. It's a general recommendation to pop it in neutral as soon as you start it up because in neutral it will fill the converter so it won't slip when you put it in gear.

Lockup transmissions have a different input shaft spline count and design so non-lockup converters won't fit in a lockup trans. and vice versa. They take the same fluid whether lockup or not. I'm out of idea's on the drain plug.

The last time I looked at a rubber gasket at NAPA it was a cheap POS that was just rubber, no steel inner core support like the good one from the dealer. If the one from NAPA does have a steel inner core that suggests they have improved them and it's probably good to use. If it's limp rubber don't use it.