If a mortgage is over a long period of time your might not win on the high interest with the credit union.
Clearly a mortgage is cheaper, but they won't give out a short term mortgage, the best way to do it is take a long term mortgage, at a low rate, adn pay it back faster.
Would your ex accept monthly payments? If you offered him extra - as in interest.
I presume the title to the house has been transferred to you and you have a legal document from him stating he has no longer has any legal or financial interest in it.