Presuming those are people to the left of the 'tank', and some sort of chimera/Russ variant on the right, that is a huge vehicle, larger than a Baneblade. You could use one of the Imperial superheavies at
http://www.forgeworld.co.uk/Warhammer-40000/Imperial_Guard/Imperial_Guard_Super_Heavy_Tanks?filter_reset=1 as a base for the conversion, but they'd be horribly expensive, so I'd recommend using one of the Baneblade variants instead and not worrying about the size. Or if you're really poor, look in local toy stores for a toy tank of the right size.
As the tank is in cover behind a wrecked building, you can't see much of it. Seems to have some sort of auto-loading mechanism? Makes sense considering how huge and heavy the shells must be.
Will it be just a display model, or do you want to play games with it? Definitely worth looking in the Apocalypse rulebook for some existing vehicle your vehicle can count-as.