We assume you're using a starter box to turn the engine over at a good rate to start it.
If fuel isn't flowing without blowing through the pressure line then you have a pressure problem. You'll need to check the exhaust seals on the exhaust header, and the connection between the pipe and header ( a piece of silicone tube, or with the one piece pipe set up there is a silicone seal between the pipe and header) Those need to be examined to make sure there are no rips or tears anywhere.
Is the fuel tank seal on the fuel lid look good or is it worn or missing? Hopefully the fuel tank is okay, but you seem to build pressure okay when blowing in the pressure line so thats a good sign.
As mentioned earlier make sure the carb setting on the main mixture screw is set out enough and the base speed needle is set to make the carb rich to start. Also when you blew the fuel up to the carb you should be able to get it to bark a little if there's fuel at and in the carb to the spraybar.
Check the carb seal and tightness where it mates to the engine block intake. Make sure its tightened there. Since we don't know the status of the carb itself, can you find a similar carb to replace the present one to try and see if that works better? If you can it may help.
Let us know how you make out. Eventually the tips here will get it running for you. Airleaks are the usual cause of fuel flow problems.