Are you using the Hyperspark Distributor, coil and ignition box as well?
Be really sure you have good grounds. Like Head/block to chassis and battery and battery to chassis.
Just did my 69 over the winter and I love my Sniper EFI. I can drive the car dead cold, maintain high idle until warm, start it hot and if it sits for a week or month it fires right up and I can drive off. You'll be happy but prepare to learn some stuff to dial it in.