Can't help you much with determining whether it's a 305 or 350. Some of the newer blocks had 305 or 350 (or whatever variation stamped on them.) Blocks are about the same size. Here is what I found on the web (edited.) "Only real way to tell is get the suffix code and casting # off the block and look it up. Suffix code is on front pass side of engine, in front of head on machined pad. #'s are stamped into pad. Clean grease/dirt off with carb cleaner, wd-40, etc and a rag before looking. Casting # is on rear driver side of block, on flange that transmission bolts to. #'s are raised so a little easier to see, but will still need to clean it good. Once you have the #'s go to
www.mortec.com to look them up and see what you have."
Don't assume the oil pan is okay because someone changed the oil. Typically, they thread a tube down the dipstick hole and pump it out.
I searched my computer files and did not find the wiring diagrams. So I guess I deleted those before I switched to a new computer. Sorry 'bout that. However, the good news is that the 305 (5.0 liter) and the 350 (5.7 liter) are so similar in design and operation, there is a ton of information about them on the Web. Both engines have been around for a long time. If you ever need to replace the engine, the Jasper MARINE rebuilt short block goes for about $2,500 and carries a 1 year guarantee.
Based on my own experience running a 25 ft boat I/O with the 305 in a saltwater environment, and working on 305's and 350's as a shade tree mechanic, here are the things you need to consider and watch.
Based on your location in Arkansas, I assume you will be running exclusively in fresh water, so there is no great need for a closed cooling system and heat exchanger. However, if you have a closed cooling system whereby the engine coolant is cooled by running raw water through a heat exchanger, your engine will last longer because the chemicals in the antifreeze mixture helps retard rust and scaling.
If you don't have a Racor fuel-water seperator, install one. This will keep water out of your fuel and make for a better running engine.
Occasionally run a concentrated mixture of Sea Foam or other decarbonizer through your engine. You'll need an external gas tank hooked up to your fuel line, which is easy to do. Follow the directions on the can for decarbonizing. You'll probably be amazed at how much junk comes out of your exhaust. If you want to keep the engine and carb in really good running order, add Sea Foam at every fill up. Also, if you are using ethanol-laced fuel, and the fuel will be sitting in the tank for extended periods of time, add a fuel stabilizer like marine Sta-Bil, or Startron.
Typically your caborator will be the source of most of your running problems. Learn how to remove it and clean it, and install rebuild kits. Depending what type carborator you have, will make this task easy or complicated. Personally, my downfall has been the Webers.
Make sure your coolant temperature gauge and oil pressure gauge are working properly. If you don't overheat the engine, and the oil pressure stays within specs, that engine will run for a long, long, long time.
If you have the standand point-breaker ignition system, consider replacing it with a sealed electronic ignition system.
Regularly inspect your manifolds and risers for leaks. If you ever have to replace them, take your time and use a lot of PB Blaster and heat. You'll still have a good chance for snapping off a bolt, so when you replace them be sure to use liberal amounts of anti-seize grease on the bolt threads.
Inspect you OMC outdrive regularly, especially the bellows connection - a broken bellows has sunk a lot of I/O boats. Check the I/O oil regularly for water incursion.
That's about it for a Sunday morning. If Makomark is still monitoring this thread, I'm sure he will jump in with additional observations and advice, since he is much more experienced than I am.