1.- Do I need to block with JB weld the original holes in the manifold
2.- Do I need to block the pulse hole in the walbro carb
3.- For the external pulse line I now I need to make a hole and install a nipple in the carb but can I do the other hole in the plastic manifold (Instead of doing it in the engine)

1. No, it will be blocked by your home made gasket
2. No, it will be blocked by your home made gasket

Same answer for both!

3. Remove the side plate from you carb and install a pressure tap in the unused boss. You can make this out of brass tubing or a tap off a junk carburator. Drill the hole small and drive the tube in. See the attached photo for how the factory does it. If you have a similar carb that has a pressure tap side plate just swap them.