Guess it all depends on what you want, what your budget is, how much time you have to swap everything. If you have more money than time, go with the turn key. If you have more time than money, go with a long block.
I think I'd go for something like this. I think this is what you posted in post #2.
https://www.summitracing.com/parts/n...1432/overview/
Buy an intake and headers or exhaust manifolds for it. Swap over you accessories - distributor, carb, power steering pump, alternator, you can make a judgement call on the water pump. I'd leave the TH350 unless you are swapping to a 700R4.
One thing I notice with the turn key engine you have listed there is the serpentine belt. It does have it's own alternator, water pump, and power steering pump. For PS, you would likely need conversion fittings for SAE to metric threads. Could swap to a later steering box with metric threads. Would be a nice upgrade if you still have a 4 turn box.
How much the 305 goes for is going to be very much area dependent. Check craigslist and see whats out there for comparison.