My transom was in good shape but I too wanted a solid transom, so had Don Herman fill in the cutout and install one of his brackets. I did the rigging on it per his advice, which is to run all the motor rigging thru the transom into the flotation tank (and fill thru-transom pipe with RTV!) and then up thru the swim platform, which leaves a clean platform for diving without all the hoses running across it. I kept the existing baitwell under port and added some plumbing to keep it functional after flotation tank covered up inlet/outlet port.
You have to be careful with a bracket as it tends to screw up balance and self bailing by moving the motor weight aft. You'll also need a stern-lifting prop, probably a 4 blade. I'd recommend a bracket with the biggest flotation tank you can get, which is probably the Hermco, although it's 30" setback is more than I need. The porta bracket probably has less setback but also no flotation. I believe Strick used 24" on the one he made, which would be enough for my motor. Don might be able to make you a bracket with less setback.
Also have to be careful with motor weight. I increased motor weight from 300 to 429 lbs + bracket, and my waterline went down about 1" to where it's just barely self bailing. They raised the cockpit sole in the later Seafari, so yours should be a little better. The Merc 150 should be ok; the only 4-stroke I would consider is the 140 Zuke, as it's about the same weight as my E-Tec. All the rest are at least 50 lbs heavier based on dry weight numbers, and I'm not sure they even include weight of the cowling.
Here's some pictures of my setup:
Here are some before/after pics of waterline (both are with full fuel tank):
