Basically, an Olds 307 really sucks. It will frustrate you to no end and never deliver good performance. You'd be better off finding a 1970's Olds donor car (
Delta 88, 98, Cutlass, Cadillac Seville from 76-78 or 77-79 Pontiac Trans-Am)and getting either a 350 or 403 to bolt right in place of it( Expect to pay from free to $500 for the donor-shop around!)It's not like you have a 442 or
H/O and the 307 actually adds to the value. BTW,the manifold is likely leaking due to corrosion which can plague older aluminum engine parts that have not seen proper cooling system maintenance.
As far as carbs are concerned, I was having this discusion in reverse with my 60+ year old neighbor in regards to fuel injection. (He fears it, I don't.) Both do the same things, just differently. Carbs are easy to fix once you understand how they work. Basically, every car needs a higher idle on cold start than when warmed up, all cars have differing timing needs dependant on RPM and combustion efficiency. Fuel needs differ based on load, etc. Carbs are actually cheaper to mess with than EFI, and not too hard to understand. Get a good book on the model of carb you want to play with and read it several times through. Basically, you'll likely have a E4ME Rochester Quadrajet stock on your 80's Olds 307 unless you de-computerize it. The E4ME uses a TPS signal, MAP sensor, O2 sensor, CTS signal and a Barometric Pressure sesnor that tell the computer how to adjust the Air/fuel ratio in the primary circuit via a Mixture Control Solenoid in the carb. It should also have ESC ( Electronic Spark Control) that funcions trhough the distributor and a knock sensor, just like an EFI car. The mechanical parts of the carb are limited to the needle and seat and the float that controls them and the secondary jetting and metering rods. It's all very easy to fix once you understand how.
Getting back to an engine swap, I'd go without the computer if you have no local emissions inspection program. However, I would still run a properly set up Quadrajet carb from a 1966-1986 non computer application. I'd also run a 1974-1986 non computer HEI ignition for it's simplicity and low cost ( it would be likely both of these items will be stock on whatever donor vehicle you get the parts from.). Plenty of parts exist to set up both of these parts cheaply, and even with a new intake you will likely come in way under $300 for your complete ignition and fuel systems- including wires, fuel pump, lines, air filter, etc.
For the floor, you have a few options. Depending on the size of the hole, you could fiberglass it, weld in a piece of new steel cut from a leftover panel or replace the whole floorpan. Reproductions are available from Year One, as are many other popular wear items like weatherstripping, carpets, etc. Ebay can also be a valuable resource. I picked up a full set of four factory service manulas for my 85 Cutlass for around $15-including shipping- a month ago. You'll also want to try and find a local U pull it type junkyard, as 80's cars are still relatively plentiful in these places. I got my 3.23 geared open rear axle for $65 with tax 2 weeks ago at my local yard, and have scored things like a almost new Edelbrock Performer intake for my Chevy 350 for as little as $15 with perserverance. You may also wish to try Craigslist.com for the donor car and parts. Also, try the Bargain Trader, Dibit.com,Yard Sales, swap meets, driving around poorer areas to see what's dead in people's yards, etc.