Considering Holley sniper EFI for my 1969 340. Thoughts from people who have that set up?

There are several ways to accomplish a steady supply of fuel for EFI - and incidently, they can also help with the issue you mentioned about fuel delivery taking too long with a carb. Just a matter of what you set the fuel pressure at. All require adding a return line. I have used all these approaches on one project or another.

1. Lazy option - just stick an inline pump on the existing system. These sometimes have issues on the suction side and will have issues with fuel slosh leaving the pick up uncovered. I have not used Hydramat; it's supposed to work well on slosh issues but costs a lot.

2. Stick an in tank fuel pump on the end of your sending unit. Helps avoid suction issues but still has slosh issues.

3. Surge tank. Less slosh issues but you have the expense and complexity of two fuel pumps.

4. Weld an external sump on the fuel tank with a nearby external pump. This is what I have now. Simple and the main downside is a little less ground clearance. I didn't use an expensive aftermarket sump kit - I welded a cake pan to the fuel tank.