Anything I should be looking for when inspecting the drive couplers? They look OK from what I can see but want to be sure before I install the engines.

If they are aluminum couplers, throw them out and order steel ones. If they are steel, you can clean them up and look at the splines for shifting, or cracks at the base of the splines but it is hard to see that. Best bet is just to replace it/ them if they are near 10 years old. Also, it is nearly impossible to say how long one will last. Some blow the rubber and you can't inspect that, some blow the splines. My last coupler lasted 12 years but had a buds fail after 5 years.If in doubt, replace now.......

look at surface area where rubber bonds to metal, if you see line or shiny surface the hub has spun. the bond surface should flow from rubber to metal. sometimes you can see little rubber balls/pellets from breaking loose.

The splines where the drive input shaft goes should not be at all sharp.

Less than $300 bucks each now while their out. If their older and have 4-500 hrs on them plus what Griff said, Replace them now! How much will it cost you to do it later? Don't forget to include down time.

After a closer inspection one of them is showing some wear on the splines; the other looks to be a little newer, possibly replaced at one time?? For the piece of mind I'll replace them, not worth the gamble, really hate to have one fail at the Lake... would kinda take the fun out of a weekend! LOL

Stock merc or aftermarket better to buy, if aftermarket, what brands are best?

Had mine go out, was going around 65 mph having fun went to stop by my friends place slowed down to idle and she cut loose. Replace if you have engines out, i went with steel splines

I am sure some on here will say not to use them but when I replaced my 310 hp 454 i also put in a new Sierra coupler behind the 496mag ho. 3 seasons so far.. 25ft VIP Vindicator in salt.. No problems..

Chris