I am troubleshooting pump performance on a 58hp submersible Flygt pump. I want to install a pressure gage on the discharge pipe but the discharge pipe is flexible Goodall hose and is directly connected to the check valve. after the check valve it is 4" hard pipe. Should I drill and tap directly into the check valve? Any Ideas...

Since the pump is totally submerged your suggestion seems to be good except wouldn't the check valve give erratic or inaccurate readings? In addition, would it make any difference if I cut a hole and welded a pipe coupling to the pipe and installed a gage from there? or, is drilling and tapping the way to go. I am pumping discharge mine water.

Unless you are using the pump to move water long lateral distances, the vertical head you are pumping against will be 85%+ of the actual head the pump sees.  Put your guage anywhere convenient, above or below the check valve.  What is the nature of you problem??  If it is low flow at the discharge, your impellers are wearing. Ignore theoretical differences in water SG, you are pumping normal dirty mine water, not slurry. The impellers can be adjusted by a competant millwright but by the time you've pulled the pump to change it out, its generally more effective to fully dismantle the bottom end and overhaul completely.

And if you think the bottom end is wearing, let the pump run continously. Lower end wear usually involves some wear on the seal between the motor and the pump. While running,  there is some force applied to the seal that prevents water passing the seal.  Turn the pump off for a few minutes,  water by passes the seal, and when you restart it, bang,  you need a motor rebuild as well as a pump rebuild.

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