Why don't you just put the original manifolds on it until you figure out what you need for the headers?

alt
 

Cash you are doing a nice job but some things you are making way more complicated than they are,the 1 1/4 hose is a suction line that is available at any marine store,they don't make heater hose that big.Also where it hooks to the out drive their is a rubber bushing you need to change hose size ,a middle 80's 350 mercruiser has the alt mount you need.in fact it is plumbed up the way you need to be.I have a alignment tool you can use if you pay the shipping

I have called west marine. They don't have it. I have called Skipper buds. They don't have it. I have checked weaver, napa, home depot, ace, farm and fleet, and three industrial hose facilities, and one large truck repair warehouse. They don't have it.

I'd be up for changing hose size, but where can I find fittings or adaptors? Will I risk ruining my engine by going to a small hose size?

Cash. The hose size is actually a 1"thats what fits on your power steering cooler that is what you need the adapter piece is a mercruiser item,so if you ask them for 1' marine hose they will have it every mercruiser motor in the country uses it the size that fits on your steering cooler is the same size that will fit that thermostat housing.I have 5 merc motors and they all take the same size. JOHN

It is most definitely NOT 1". Type in powersteering cooler, mercruiser. All of them, for GM v8s, are 1.25" inlets and outlets.

So I'm doing the bellows. How does one remove the shift cable to replace the bellow on the shift cable?

hose
 

It was a typo.The hose is a 11/4" that's what we have been talking about.Like in other post.JOHN

It depends on if it is a factory pinned one or if it has been replaced before. My suggestion would be to replace with new cable if you are replacing all the bellow. It makes sense. One can remove the shift many ways but here's one of them
1. disconnect at the at the engine and pull out through the transom. This could require a 2 man job to reinstall and rought correctly if your are running the y pipe.

cable
 

You also need a very deep 9/16''socket to unscrew the outer shift cable out of the gimble helmet after you remove the shift fork and inner cable.

I got the shift bellow in, but the main bellows... no ****ing way without removing the bell housing. I dicked around for over 2 hours. I'm going to try to find someone who can do it locally, I give up.

Okay, I have the cooling system worked out. I found a 1 1/4" hose at a hydraulic hose place. Also using a plastic barbed elbow, with clamps, but the issue is the hose that goes from the PS cooler to the thermostat housing is hitting the alternator. I'll have to make a spacer or something.

Here's another big snag: I ordered new fuel fills. They're gorgeous, and the right hose size. However, the DIAMETER is smaller than the old ones, and the three mounting holes do not line up. Does anybody have a source for fuel fills that shows the circumference or measurements of the mounting holes???