Forget the 391 trigger Brad is right get a 390 trigger or trade it for a 390 that has the bent carrier. Then sell the 391 trigger for 225.00 is a fair price for a release. You could always just remove the hook and have a pull trigger. Lots of the 391 guns had the straight carrier and had to have them replaced with a bent carrier after a few thousand rounds because of feed problems. The 390 triggers were metal the 391 is plastic I have both but prefer the metal housing . If you decided to return to a pull trigger do not buy a sear I will give you one for free.
I wouldn't try to convert the release trigger back to a pull. There could be
other modifications that were made besides just replacing the sear with a release hook. On Spear release triggers,the spring and pin that allow the sear to move are replaced with a bar that makes it immobile. That would have to be
changed. There could be modifications to the hammer, too. Either keep the release trigger for the future, or sell it.