'79 225 engine -is it a hydraulic lifter engine?

An original 225 engine in a '79 is a solid-lifter engine.

Officially, the first year for hydro lifters in the Slant-6 was 1981. A small production test run of hydro motors was built and installed on regular production vehicles during the 1978 model year—not as a special option or anything, just some people who bought 1978-model Slant-6 vehicles happened to get a hydro motor; particulars were given in a large TSB. Other than that, no, 1979 and 1980 were not years when it could be either. I think some people are mixing up the midyear-'76 changeover of the 225 engine from forged to cast crankshaft with the '81 intro of hydro lifters.

The hydro-lifter engines have a different valve cover bolt pattern, so that's an easy visual ID for a factory hydro-lifter engine, but the hydro valvetrain parts will fit just fine under the solid-lifter valve cover, so if someone mixed and swapped parts in the past, glancing at the valve cover won't necessarily tell the tale.