I did a similar conversion. I have a 1969 Penn Yan Explorer. It had a 160 model, 250ci straight six, with am older MC1 drive - it had a fitting in the gimbal housing for trim lines, like yours - I had plugged it at some point, drilled out the bottom of the gimbal housing and installed the 2 into 4 trim connector - you may want to think about that if your housings are good. Regardless, I got my hands on a complete MR gimbal assembly, from an 85 Sea sprite with a four cylinder, which included the engine bell housing, inner transom plate, gimbal, etc., everything but the drive. If I remember correctly - I had to trim the hole in the transom slightly, and maybe some of the gimbal housing bolt holes as well. Here's the thing with the engine components - your existing bellhousing will not bolt up to the MR/Alpha One and later inner trasom plate. Your mounts are further apart than the MR mounts, so you would need the later bell housings to bolt your motors up to the transom plate. Then, the exhaust tube is different - yours goes in between the rear mounts, and the later goes outside the mounts. You will need to get the later model exhaust tube for a four cylinder. Next you have to get your exhaust riser to mate with the tube. I initially cobbled something together with aluminum automotive exhaust pipe and silicone elbows, howevevr, I was able to make it right with aftermarket risers. I needed to use one of the four cylinder risers that comes off the exhaust manifold at an angle. I was using an OSCO manifold, so I had to use a riser adapter too. Ultimately, it all bolted up using a stock Mercruiser exhaust bellows hose. I then bought an SEI outdrive, plus their trim pump and cylinders, and basically had a new rear end hooked up to my beloved straight six chevy. As far as the outdrive goes, the input shaft is a longer version than yours. If you install a later model drive, it should have the correct shaft already.
Ran it this way for a few years, then blew the head gasket at the #6 cylinder last summer. This spring I re-powered with a 2016 Mercury remanufactured 4.3 4v 225 horse, but that's another story...