what the best way to weld this crack any suggestion? 

Best bet would be getting a certified welder do the repair 

Hard to say, just looking at this one picture

Take about 20 pics from all different angles. Send them to the factory and let them decide the best way to repair. ESpecially if it's structural where a failure could kill somebody. 

if that is structural, you HAVE to take it to a certified welder.  Need more pics

A certified boom inspector should do the welding.

That is a really bad question you ask. Looks like a support from the pedestal to the frame pump chassis frame.

First and foremost seek advice from the manufacturer not some backyarders off an internet forum.

If the manufacturer cannot offer you a remedy (they should however), seek out other same brand pumps of same model and see if they are designed same or are there imporvements and then implement the improvement to your machine.

If there are no improvements which have remedied this problem or its a freak break, speak to real mechanical engineer for advice - who has a proven history in cranes and hoists and get them to request a repair weld procedure.

Sorry but a patch up job will NOT suffice and this break is not a cut and shut job. It has to be repaired and reinforced (even depending on your country documented and signed off). Worst thing is having to do a repair twice. Do it properly first time.

THANKS FOR THE HELP  WILL CONTACT ALSO THE MANUFACTURE I WILL ASK HIM ABOUT THE BEST WAY TO REPAIR ITand i will ask about the best welding rods also