I've not used the molasses thing, but have used phosphoric acid, vinegar, and oxalic acid for rust treatment. I'd recommend agains pool acid (known as Muratic Acid, which is hydrochloric acid...HCL) it's much more reactive and thus requires considerably more care, and harder to neutralize and dispose of. I'd typed out a long discussion of the one's I used earlier today, but the site had a hiccup this morning and ate it.

Oxalic acid is the primarly ingreadiant in CLR, a product marketed for household use to remove rust and mineral scale. As a practical matter that may be the easiest/best/safest product for a one time/hobby project like a block. If you've got facility or vendor support, the more aggressive acids might be viable.

Products like Ospho, naval jelly, Rust Mort, and most other commercially available brand name rust "converter" products are phosphoric acid based materials, so if you buy straight phosphoric acid you can avoid paying for dyes, advertising, packaging, and all the rest of the convenience costs.