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

Start with the fuel system. Check to see if you can buy a complete EFI tank with build in fuel pump for your make and model. If you have a late model Barracuda you shouldn't have any issues, the early ones might be a problem.
I wouldn't buy a Sniper until you know that you can buy a tank with an internal pump.
The Sniper will work best if it is paired up with a Hyperspark distributor and a CD box.