can you install a rod backward on a slant six?

Yes, you can and you shouldn't. What year engine are you working on, and if it's a '63 engine, is it an iron or an aluminum block? On all 1960-1962 engines (iron and aluminum) plus 1963 aluminum engines, the connecting rods get installed so that the squirt holes face away from the camshaft. On all 1963-up engines except the aluminum blocks, the connecting rods get installed so that the squirt holes face towards the camshaft.