I don't mean to be a parade rainer but you got excess time on your hands this winter or just feeling board?
I know what your after because I did it but jeez your looking a big mountain in the face on this one.
Here's some thoughts for you.
They already have that, pretty much; Pilatus Porter, Cresco, PAC 750 XL, Single Otter, Short Caravan, Kodiak, Soloy 206, TurboTech 350 horse piston (bad, but it's there). So you're kinda re-skinning the cat and the poor thing is kinda wore out now. You're in the 6 to 10 place world and most of those have the option of hauling more if necessary to make more money.
If you're gonna do it you might as well look into a 207 instead (which brings you even closer to a Caravan) to offset the added operational costs. Might as well haul more if you can for the same engine purchase costs, maintenance costs, fuel burn and certification investment.
You are figuring the cruise burns. We both know that it isn't going to be throttled back on the way up. It'll be pulling every bit of power I pulled in some tired -20 Porters and Otters and will burn the same fuel. Probably 40-45 gallons an hour average holding it to 680 in the climb.
Overall -20's are gettin' old and tired. I've done cheaper hot's on my -34 than you can do on a -20 due to the air cooled vanery. Watch using the old shit 'cause it looks cheap on the front end. Pre-Century Garrett's are cheap too and there's a reason!
The Cessna airframe is fairly durable overall (my 180 has almost 9000 hours on it and looks good) but watch putting lots of pony's on them and working the snot out of them. Those rivets start working faster than you think.
I used to fly jumpers in a 195 with a 450 horse R-985 on it and we had some issues. Not major but the occasional re-buck of some rivets.
You may look into a Brazilian Cessna 195 that had a Garrett put on it. If you can find them they might give some input into problems they had either up front or long term. I saw pic's of that thing like almost 20 years ago. Probably crashed it by now, looked WAY TOO FUN!
There is a 400 horse Lycoming IO-720 conversion certified on Cessna Ag Trucks and Ag Husky's. I don't know the finer points of firewall differences but you might look into that as your possible H.P. instead. But like I said, if you're going to 13,500 with this thing you'll be down to around 350 at the top anyway, less if it's wasted.
If you're just looking to do this for fun, cool. But if you want to work something and make money without pulling your hair out with the feds and spending an ass load of money on certification for a VERY limited market sales potential, think about this. Take all this time and money and buy a Caravan or Porter and use the rest of the money to go advertise for tandems, sell your ass off and crank some numbers next summer! Pay the bills, have something you can sell to a broad market and spend the rest of the time on vacation.
But let me know how it goes.
My 2 cents.