Blowm head gasket

Sounds like you messed with something for no reason. Not sure why everyone loves to ditch the Fmu, but many experts still use it.
Just put it back the way it was.

If anything, i'd look into ditching the t rex and use a 255 pump in tank. But still, check with guys with superchargers or experts before you do that.

What mass air are you using? Usually going with larger injectors and no FMU requires a dyno tune / custom chip to get the air fuel ratio right. I would also upgrade the stock in tank pump for a 255 or 190. I run a 255 in-tank along with the T-Rex on mine.

Lets see...Is that timing 13 degrees total timing or base set at the distributor. If that is your base that would be 28 total degrees at WOT with 10psi of boost which would be way to much timing with 93 octane and you did not say if you had something removing timing under boost. Do you have a Fuel Pressure gauge because with with the FMU you would be at 70psi+ @ 10-12psi and no way I can see your fuel pumps keeping up. Now a switch from 30lb injectors to 42lb injectors is a better move and pitching the FMU is a correct method if you have a tuner (PMS,Tweecer,Chip...etc.) I really think your T-Rex could not keep up because you need a 255lph pump. I understand I am asking many questions but 42lb injectors will support 550 rwhp. The backfire was either before the head gasket which was a lean condition probably and if it was after the gasket the water caused the lean pop.