I remember when they were slugs big enough to count as titans...
I found a thread on here where a guy used a Tyrant Guard kit to make a batch of 3 Biovores for next to nothing.
Blimey.
I agree that the exocrine
ST do look a bit weedy and odd compared to the bulk of the thing. I think that if you could switch those with the walking arms, so that the latter were the forelimbs and the former the mid limbs, it'd make a big improvement. Kind of mirroring the smaller mid limbs of a gaunt, or other nid creatures with certain loadouts. Alternately, see if you can slot a carnifex's crab claws, or a trygon's larger - but short and thick -
ST, into the front sockets.
Personally, I think the problem with the head isn't that it's too small, but it's not 'underslung' enough, if you understand me. The main mass of the neck is set pretty low on the model - almost sprouting out of the inside surface of the ribcage - and then sticks up and out. I think it'd look better if there was more mass behind the head rather than below, like the biovores you're trying to emulate. Might be relatively easy to sculpt, with a little bit of niddy texture.
Otherwise, not sure. If you want a bigger head, what are your choices from the range? I dropped out of
40K before all these bigger-than-a-fex kits were released, so I don't have much frame of reference.