The basic exchange should be straight forward.
Items to look at would be transom assembly hole (cut out), take a large piece of card board and tace it out of the merc boat, measure hieght from bottom also. check to see if the hole needs to enlarged in the omc transom.
The motor mounts may not be in the same location so check that (stringer location). ford vs chevy
Bolt holes thru transom for mounting. the bolt pattern may be slightly different, check that also.
The merc short shift cable is different and will need propper adjustment before use, repost when at that step and we can assist. Also the routing is specific and if not routed correctly can cause rubbing on the coupler or kinks. Be very careful when handling the short shift cable or you WILL be replacing it......
Alignment ( gimbal bearing to coupler) MUST BE DONE before out drive is attached!!!!!!!!!!!!
Make sure you get all the wiring from the guages back including the controll box, trim pump.
The chevy is a 5.7 350, the ford is either a 5.0 (302) or a 5.? (351) The firing order is different so make sure you understand the suttle differences. Also the merc should have electronic ignition, the Omc would have points.
While you are at it, replace the impeller in the merc outdrive...
Use good outdrive oil..merc hi performance is my recommendation. The drive holds ~ 36 oz, follow proceedures to be sure.
That is all I can think of for now......