When relocating the battery up-front, here is how I set up my grounds:
I run a large (2 or 4ga) ground cable from the battery NEG terminal to the front spaceframe of the car's body outside of the front trunk compartment. I usually connect it to a hole I drill near where the factory sway bar bolts up to the subframe rail in the 84-87 cars. DO NOT connect the ground cable to the front suspension K-member as this piece isn't part of the vehicle's spaceframe (and there's a chance it could not be grounded very well to the vehicle's spaceframe because it is bolted onto painted surfaces from the factory).
I also run a smaller (8 or 10ga) ground cable from the battery NEG terminal to the sheet metal in the front trunk compartment (usually one of the studs for the jack storage bracket).
Back at the engine, I connect a 2 or 4ga ground cable from the block to one of the thicker metal plates that are welded to the Fiero body that the front of the engine cradle bolts to. Do NOT connect to the cradle cross bolt. Drill a new hole into the plate that is welded to the Fiero body and connect it here using a nut and bolt.
I also connect at least one 8 or 10ga ground strap from the cylinder head or transmission case to the other cradle mounting plate that is welded to the Fiero body.
Make sure ALL connections have been sanded down to bare metal to remove paint, rust, corrosion, etc before bolting on your ground wires. Apply some dielectric grease to bare metal surfaces to prevent future rust and corrosion before making your final connections.
I DO NOT recommend running a ground cable all the way from the battery back to the engine block. This is unnecessary and can add extra resistance in the circuit if this is your only means of grounding the engine to the battery. If you do run such a wire, I recommend 2ga welding or automotive grade cable.
I also do not recommend grounding the engine cradle or the front k-member. It is unnecessary and unneeded.
Make sure all large ground cable connections made to the spaceframe are done so at thicker points of the metal (like welded on mounting pads or a few layers of sheet metal that have been welded together by the factory) and not just to single thin layer of sheet metal.
For the POS battery terminal connection, I run a 2 or 4ga cable directly from the POS terminal to the starter. I also like to run an 8 ga wire from the battery POS terminal (protected by the correct size and length fusible link) to the large, 12v + constant power wire feeding the radiator fan relay. I don't sever the radiator fan relay's power wire from the rest of the Fiero electrical system when I do this, I simply tie in an additional 12v + battery power feed directly from the battery to it. Doing this helps boost internal Fiero electrical system voltage by taking some of the load off of it put on it by the radiator fan when it turns on.
EDIT TO ADD: You still need to run a POS (+) wire from the starter terminal you connected the large battery cable to, up to the factory Fiero power distribution block near the C500 connector. I recommend using a 6ga wire for this.
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[This message has been edited by Darth Fiero (edited 11-04-2014).]