Craig,
yes a pic would help!!
I've attached a (fair) scan of the standard 'battery TAV' page of the P300 parts manual. I would think this would be identical for your Silhouette (or at least pretty darn close).
If this were your arrangement, the key would be removing part # 59, a retaining bracket held by a single bolt part # 61 with a washer, part # 60. The bracket runs along the long side of the battery facing the center portion of the forward boot. The bolt needs to be unscrewed quite a bit before it clears the base of the battery and you can lift the battery out - newer batteries usually have an attached strap which makes this part easier. Of course you'd first need to remove your battery cables from the battery ... :-) I've never had to 'mess' w/ part #68, the rear bracket ...
Sounds like you have something different. Can you post some pics of your arrangement?
Regards,
Bryan
'74 P250S S/N 15622
Indianapolis Indiana USA
blissinindy@yahoo.com