The solution became obvious when I realised I’ve already been using my basilisk models (I have 3) as proxies for other IG heavy support options. After all, the all share the same Chimera STC chassis.
Each of my basilisks have no ‘front shielding’ (the front plating that would protect the crew from the recoil/blast of the big gun) which is the way I like it, so I decided that although one had an earthshaker cannon already fixed in place, the other 2 I could easily remove the gun barrel and trim the length of it to suit my purposes (cutting it to a midway length between what one normally gets between a basilisk and a medusa). So my medusa proxies were solved. They're both as long as I tell my opponent so!
Basilisk / Medusa
I then realised that I could apply 2 small magnets to each end of the gun supports, and using the bits I had prepared for my Valks/vendettas, these are the results:
Manticore (Fire away!)
Whirlwind. I mean, 'counts-as' Hydra!
Obviously when I have the time to prep a set of twin-linked hydra autocannons for my Hydra, I would, but for now the Valk rocket pods I think are a pretty good proxy for my 'Hydra'.
All in all, I think this weapon-swap method is a pretty easy, workable, and convenient solution to proxying, without relying upon buying many different flavours of tanks.