No 790 mt psu is not proprietary. It is standard ATX psu form factor with 24 pin ATX, 4 pin cpu, and 4 sata power connectors. dimensions 15 cm x 14 cm x 8.5 cm
Is the PSU in the 790 MT i5-2400, 16gb ram, gtx 1650 proprietary and can i put it into a microATX case?  
Can I put the PSU in my Dell OptiPlex 790 MT into another case?