Small block 727 transmission tube and dip stick...

If you want OEM get a stock dipstick tube and put one of these on:
Transmission filler tube repair end transmission dipstick tube repair
It comes with the top-hat style grommet

The original OEM style which uses an O-ring is inferior. GM used this style of grommet as well as later Chrysler transmissions.

FYI you can install it a variety of ways. Weld, braze, JB-weld, epoxy, etc. It's not critical.

Also FYI this is exactly what I have on my car.