Did up this conversion for a friend today and I thought I'd share how to do it for those who don't want to shell out for the FW weapon pack.
If you're OK with using an ork big shoota, here's what you'll need:
1) Goliath model with the two separate lower legs
2) Goliath rivet cannon and arms
3) Ork big shoota bit
4) Sharp hobby blade
5) Pin vice, drill bit, brass rod, super glue
6) Drum magazine from a space marine or ork gun.
7) Optional: Left big shoota arm with ammo belt, big shoota ammo backpack.
Step 1) Assemble the Goliath except for the arms
Step 2) Remove the parts marked in red from the big shoota and rivet cannon bits:
Keep the rivet cannon piece, toss the big shoota parts you cut off.
Step 3) Drill holes in the parts where the red lines are, pin and glue them.
Step 4) Drill the right hand and wrist stump, pin and glue them. You will want to do some test fitting at this stage to see exactly what angle you will need to drill into the hand with because it's going to be at an angle compared to the wrist.
Step 5) Glue the right arm to the body and glue the right hand/shoota piece to the arm. Take the left arm and figure out the angle you will need to rotate it to get the flat part of the support handle to mate with the flat side of the shoota action behind the round barrel shroud.
You will need to trim the shoulder quite a bit because of the weird way they engineered the arm to fit onto the body. You may need to pin the arm to the body because there might not be that much surface area contact between them after cutting down the shoulder for just super glue to be a strong enough bond.
6) Cover up the mangled shoulder with the big skull shoulder pad and greenstuff the gap between the right hand and arm.
At this point you could cut a drum magazine off of another ork or space marine weapon to put on the stubber, or...
Optional Step: Ammo Belt
Remove the strip of the the ammo belt from the left arm of the big shoota bits:
On the big shoota you will section the lower part where I put the red lines. By that I mean you will remove the half of that piece closest to the Goliath's body and it will take 3 cuts. The first cut is down the center line and will divide that tab in half. The next two cuts are where I have the red lines on the picture and will allow you remove the inner half of that block.
Clean up the ammo belt connection point and glue it in the spot you just opened up by removing half of that block.
If you want to use the big shoota backpack you will need to trim down the gubbins on the Goliath's back until you get a good fit and then super glue it in place (or pin it, if you're nuts about that sort of thing like me).