I ran all thread through the original mount to keep aligning the first mount easy.
I moved the motor mounts back 8 3/4" as seen in a previous post with a tape measure against the firewall.
this measurement is only for a 4.3, the sbc mounts are further forward on the block and roughly centered on the 4.3, so it will work out to about ~5" on a sbc. the way to figure it out though is to do exactly what I did, mock up the cab and measure how much to move the engine.
the bars have holes centered 16.5" apart, yes.
the bars were heavy and needed some support so I tack welded the mount sleeves to the bars and the forward bar to a pipe that was stood on the ground. it was one of those 30 second solutions to keeping things aligned. its actually the remnants of the 49 steering column haha, you can see the horn ground wire on it I think.