Over-riding tpls is awful & hard to sustain. But if you HAVE to do it this extension could help. By default CiviCRM allows you to override the tpl for a given contribution page by putting the file in the path
yourcustomtemplatefolder/CRM/Contribute/Form/Contribution/
If you want to override ONLY contribution page 6 then
yourcustomtemplatefolder/CRM/Contribute/Form/Contribution/6
This extension also supports - contribution pages with a financial type of 6
yourcustomtemplatefolder/CRM/Contribute/Form/Contribution/Type6
This extension also supports - contribution pages with a campaign of 6
yourcustomtemplatefolder/CRM/Contribute/Form/Contribution/Campaign6
or just
yourcustomtemplatefolder/CRM/Contribute/Form/Contribution/AnyCampaign
for contributions with (rather than without) a campaign.
However, even better it not to do tpl overrides....
