[GeeTwo]

I can't see the picture, so it's hard to be specific.

If the block is made of steel or another hard metal, and you have five threads engaged, you're getting as much holding power by tapping as you would by using a through bolt. Your block is 1/2" thick, so if you're using 1/4-20 bolts, the bolts would touch when each engaged 5 threads; this would not be optimal. If you're using finer thread bolts, it would just be a matter of trimming the bolts to the right length: no less than 5 threads engage, but less than 1/4" will enter the block. If using an aluminum block, definitely go with the through bolt.

Depending on how much vibration and variation of load you have, you may want to use lock washers on the bolts; I prefer the ones with the interior teeth, as the grip is at least as good and there are no sharp edges exposed. Given that you were going to use lock nuts originally, this is probably the case.

Another solution if you have an aluminum block might be to use a coupling nut within the aluminum block, which has even better strength than a tapped hole in steel.