You can change the fin profile slightly, widening them a little at the aft end, along with changing the length like you mentioned to help the CP a little. Move the motor forward, slightly recessing it in the BT-60 and gap stage it. It will only change the CG a tiny amount, but every little bit helps when it comes time to add nose weight. It has a plastic nosecone, so getting the minimum amount of weight as far forward as possible won't be any trouble. IIRC, it has 1/8" fin stock. You could remove a little weight from the aft end of the sustainer by using something similar to contest grade 1/16" or 3/32" balsa. Or just dump a bunch of shot in the nose and use a D12-0 booster.

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