If you use a return style pressure regulator, you can fit an in tank pump into your fuel tank through the stock sending unit opening. Here's a few pic's of how I attached mine to the stock sending unit. If you look closely, you can see the up-sized return line silver soldered to the sending unit ( note it is pointing away from the pump pick-up). Your return line will need to be at least as large as your feed line to avoid restriction at such a high bypass rate encountered with the low fuel pressures that you need to run with a carb set-up.

I sourced the bulkhead electrical fitting from Radium Engineering and the submersible fuel line from Harmon Racing Cells.

I run an EFI system on my '68 with a surge tank up front, so the pump pictured flows at near 0 psi and at full flow as the tank fuel circulates through the surge tank up front. This eliminates the cavitation issues associated with fuel slosh at the tank during low fuel level use extremely well.