Swapped lower units on MC 1

[hws1776]

Here's the deal, I shredded the upper unit on my OD and found a complete drive for $200.

I had recently had the drive rebuilt (prior to the gears going) so the lower was good (new pump & seals).

I thought I could just swap the new upper with the rebuilt lower and even though they seem exactly the same, now I cannot get it to go into gear.

I am hoping it's something simple like not having it in fwd or just not having something aligned correctly.

I am a recent inductee into mercrusier mechanics as I cannot afford another 1100 dollar snow job.

Please lend some advice.

Old bad upper is a 71 MC1
New lower is a 74 MC1