Like yellowcanoe, I have ordered from Essex Industries and Ed's Canoe multiple times and have been quite satisfied with both. If you have specific questions that need to be answered, Ed's is probably best as you can talk directly to him or his wife. These days, I usually check Ed's site first and go to the "Factory Specials" tab at the left of the home page. That will take you to a page with the hypertext "Cleaning Out The Closet". Click on that hypertext and it brings up a drop down menu box opposite the entry "Ash Carry Handle $2.50". Click on the drop down menu arrow and it will bring up a menu list of specially priced items currently available. This menu changes frequently and I have also found that it is sometimes not always up to date, so if you want to order an item listed there it may be best to phone and check that it is still available. I have sometimes also needed special items not listed on the site that Ed has been willing to make and sell to me at a very attractive price.
For example, currently listed on the Cleaning Out The Closet menu are a cane Mad River 32" wide seat at +16 dollars (meaning $2.50 + $16 = $18.50), a webbed Mad River 32" seat at $20.50 total, a 36" ash yoke at $12.50 total, and a 40" ash yoke at $15.00 total, and a 40" stained extreme duty yoke for $17.00 total. If you check around you will find that these prices are nearly impossible to beat, even if you have the wood, material, finish, tools, and means to make and finish your own seats and yokes.
I am not familiar with Indian River canoes but looked at some pictures on their site. It would appear that a 36" long center yoke would be long enough for your boat. A problem might arise in mounting it depending on the nature of your gunwales. Aluminum gunwales are sometimes not suitable to drill holes through the inwales to hang seats and thwarts. The inwales might be curved on the top, too narrow, and/or too weak to allow this. Judging from the pictures on the manufacturer's site, it appears that the center thwart is mounted on L shaped brackets that are fixed to the sides of the hull with rivets or machine screws that go through holes drilled just below the sheer line. This type of aluminum thwart is often crimped flat at the ends with the horizontal elements of the aluminum L brackets going into the open ends of the crimped thwarts and secured to it with a pair of pop rivets on each side. If that is the arrangement on your boat, you can probably use these same brackets to mount a wooden center thwart or yoke by drilling off the heads of the pop rivets holding the brackets to the thwart ends and pushing out the rivet (if necessary) using a punch or drift.
Also check the way your seats are mounted. If they are secured to aluminum brackets that are fixed to the sides of the hull with rivets you may be able to remove the existing seat and use these same brackets to mount wooden framed seats, but you will need to check that the brackets are long enough in a fore and aft direction to accommodate the seat frames. If not, you may be able to lengthen the hanger brackets with pieces of metal or ash mounted to the bracket and long enough to bolt the seats to.
As to how to portage your canoe, I think the optimum method will depend on how you intend to use it. Your boat probably weighs at least 65 lbs and may weigh 70 pounds or a bit more. It may get a tad heavier with new seats and yokes. I have used 70+ lb boats in Quetico, the BWCAW, and on Appalachian whitewater rivers where there were solo carry portages up to 1/2 mile, but I was much younger then and would not dream of doing so now. Also, if you intend to do tripping in places like that be aware that carrying the boat up and down steep trails and threading it through closely spaced trees will be much, much harder than carrying it around your back yard. Unless you are young and strong, if you plan to do that type of tripping I would suggest double carrying the boat and I agree that the marathon method of portaging will be uncomfortable with a boat of that weight, and may be very difficult to do on narrow portage trails.
I have never used two yokes on a tandem canoe but I have seen a few set ups that looked as if they would work well. I would first see it the front of the seat frames can serve as yokes as yellowcanoe suggested. Depending on how far the seats are mounted below the gunwales, you may need to rig removable pads to the seats to position the boat comfortably on your shoulders. If that does not seem practical you could try two yokes. If you mount two fixed yokes you will need to place one sufficiently aft of the front seat frame to allow a person's head and shoulders to comfortably clear the seat frame. If you mount another yoke permanently you will either need to place it sufficiently in front of the stern seat to allow room for the stern paddler's legs, or else mount a very short yoke behind the stern seat. Two yokes permanently mounted between the two seats will limit the size of packs and/or barrels that can be carried in the boat for tripping, however. You might consider mounting one yoke permanently behind the front seat and using a second removable yoke that can be positioned further aft at the place that best balances the weight of the boat. Here is an example of a removable yoke sold by Essex Industries:
http://www.essexindustries.org/universal-slotted-yoke/
