Couple of things.
That 'fitting' at Target is for the steam-punk/industrial decor look for things like shelves and brackets and such. They LOOK like pipe fittings but they usually aren't RATED as actual pipe fittings. Beware.
Next, 275 gpm through 3" nominal pipe.
Eek.
Your flow velocity through the pipe is ~2+ times what best practices call for regarding flow velocities.
You are going to have flow erosion problems on the pressure side and probably cavitation problems on the suction side. Noisy operation, increased erosion wear on the piping, increased risk of cavitation and wear on the pump (cavitation wrecks pumps!), not good at all for continuous operation at that flow rate and flow velocity. Marginal operation even for an intermittent use case, such as fire sprinkler system.
At 275 gpm, better practice would be to use at least 4" nominal piping, long sweep elbow and not a street elbow, and still check flow rates and fluid velocity for the complete flow system. If this is a recirculating fluid system, also check for suction reservoir size and settling time to minimize possible air entrainment in the fluid.
Check your fluid system design some more, because 3" pipe and 275 gpm isn't really right.
You said you wanted to weld the street elbow to some steel plate, but then later said the the elbow is going into a round section (tapping into another pipe?). What's going on? I think there may be more to consider than just how to weld a fitting.
That 'fitting' at Target is for the steam-punk/industrial decor look for things like shelves and brackets and such. They LOOK like pipe fittings but they usually aren't RATED as actual pipe fittings. Beware.
Next, 275 gpm through 3" nominal pipe.
Eek.
Your flow velocity through the pipe is ~2+ times what best practices call for regarding flow velocities.
You are going to have flow erosion problems on the pressure side and probably cavitation problems on the suction side. Noisy operation, increased erosion wear on the piping, increased risk of cavitation and wear on the pump (cavitation wrecks pumps!), not good at all for continuous operation at that flow rate and flow velocity. Marginal operation even for an intermittent use case, such as fire sprinkler system.
At 275 gpm, better practice would be to use at least 4" nominal piping, long sweep elbow and not a street elbow, and still check flow rates and fluid velocity for the complete flow system. If this is a recirculating fluid system, also check for suction reservoir size and settling time to minimize possible air entrainment in the fluid.
Check your fluid system design some more, because 3" pipe and 275 gpm isn't really right.
You said you wanted to weld the street elbow to some steel plate, but then later said the the elbow is going into a round section (tapping into another pipe?). What's going on? I think there may be more to consider than just how to weld a fitting.
You don't want the pressure side plumbing to beat itself to destruction because it couldn't support the flow rate the pump was pushing through it. That gets expensive too. 
Etc.
