Before you go adding an electric pump, you really need to find out why you have a vapor lock problem. Check the line routing, make sure it is well away from heat sources, and possibly even insulate it if it has to run near them. You should also check the mechanical pump for proper operation. The stock pump should be plenty to feed the webber designed carbs that the Langdon setup uses.
If you do decide to run an elecric pump, you may want to consider using it as the main feed, rather than a "booster". You'll need to have a regulator, and possibly a return line as well. Mount the pump as close to the tank as possible, as electric pumps are better pushers than pullers (that's why modern cars have them in the tank). You will also want to have a pre-filter (sock) on the pickup tube in the tank to prevent any debris from damaging the pump.
