I got an inline 6v pump from MACs a while back, but when I search their site now I can only fine 12v versions. I believe it was Airtex and has a pressure rating of 2.5-4.5 psi. This won't put out too much pressure for your carburetor so you won't need a regulator. I mounted it inline near the tank and used it as you describe. I found that I sometimes had to crank the engine for a couple seconds to get the pump to start pushing fuel through the mechanical pump for priming. I'm not sure why, unless it can't push fuel through if the diaphragm is in a certain position. I also found it good on very hot, after a short shut-off condition, when the mechanical pump would struggle to pump. I could switch on the electric pump and everything would smooth out. After I'd get moving again, to get good air flow through the engine compartment, I could switch the pump off and continue on. My switch is double throw, on-off-momentary. I have it wired so it'll activate the pump either way.
Recently I replumbed it to have a parallel line from the electric pump that bypasses the mechanical one. It still works the same way but primes up every time without needing to give it a short crank. I also figured this would be a better backup if the mechanical pump failed.
My pump looks like this