On 454's putting out about 450hp with 750 double pumpers should I go back with mechanical or electric fuel pumps? Also what brand and model?

A standard Holley mechanical will easily support that hp without the potential problems of an electric. Easy choice: mechanical.
Gary

Mechanical I use the Carter H/V. The stock merc pumps should even be fine.

At what HP level does a mechanical pump just not produce enough volume? I see pictures of blower motors with mechanical pumps all the time which leads me to believe they can support 700 plus HP?

I am running a Holley 170 gpm mechanical on my B&M 250 setup. Single 1050. Carb was converted to marine by none other than Nickerson Performance.
Cost? $127.00 plus shipping.
Holley does not make a 170 gpm mechanical marine fuel pump.
I know that on some of his blower setups, he uses both a mechanical and an electric, together.
I was running an Edelbrock 130 gpm prior to the Holley. The housing developed a crack and was spraying oil into the bildge. It was a non-marine version and seemed to do the job at 130 gpm.

Carter High volume (140 gph) internally regulated and just tap a vacuum tube from a carb into the vent hole on top and run a tube up to the carb or flame arrester.
Got them on my 540's and the pressure never moves.