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If it is just the block you are using then everything should be fine, provided you use the same bits as was in your old one. You haven't given much information about what you are planning on doing! There is nothing special about the v5 block, it is just an EJ20 block. If you were going to use the v5 heads aswell then you would need to change your ecu as it would need remapping but standard WRX heads and a standard STi block is no different to a standard engine. If you want to spend 750 quid on a new ecu then that is upto you but if you are then it would be better to use the STi, internals and heads aswell.