Keeping historic links in place

I am re working an aged website with a large number of valuable links in place that need to be kept. The problem is that the website before was flat file and as such all pages ended in .html. Now I am using a plug in “WP Page Extension” that adds a .html to the end of a WP Page, but Google and others (SEOMOZ for one) don’t seem to like this and randomly report the link as a 404.

Is there anyway around this problem that is permanent and that Google and others will not have problems with ?

thanks for your time