It would seem that it might be possible to match SpaceX's Falcon 9 with a booster that could be built possibly more cheaply and with far less development time using older, updated tech?
Before the Saturn IB production lines were shut down, developing that launcher further might have been cheap and quick, but at this point the tooling, and the people with the know-how to assemble one, are long gone. You'd have to start much of the development cycle all over again, as discussed in a QA about the Saturn V. This problem is exacerbated by the fact that several different companies were involved with the Saturn IB: engines from Rocketdyne, stages from Chrysler and Douglas Aircraft (both out of the rocket business and merged and remerged with other companies several times since then).
Would a Saturn 1 have enough residual fuel to allow a landing/recovery similar to SpaceX?
For Falcon 9, there's about a 30% payload penalty for booster recovery. The Saturn IB stages at slightly lower velocity and altitude to F9 (59km and 1900 m/s versus 66km and 2200 m/s) but would have to do the landing burn on less-efficient engines, so it's probably pretty close to a wash.
Would it be possible to combine the structures of a Saturn 4B tank assembly with the "spider beam" structures that form the top and bottom of a Saturn 1, with a stretch of the tanks length to match the size/capacities of the Saturn 1?
Are you talking about replacing the first stage with a sort of stretched S-IVB here? This would eliminate much of what possible development benefit you might get from starting with a Saturn IB. The S-IVB was a hydrogen-oxygen stage with a common bulkhead between the fuel and oxidizer tanks; that would freeze the kerosene in a kerosene-oxygen stage, so it would really be a new stage design.
I don't know the exact mass fraction of the S-IVB tankage versus the Falcon 9 first stage. Falcon's stages are very light; it has the advantage of more modern structural engineering techniques.
The real nail in the coffin of this proposal is cost of the engines. The unit cost for the J-2 engine used on the S-IVB in 1968 was about US\$2.1 million, something like \$16.4M in 2021 dollars -- the combustion chambers were lined with scores of regenerative coolant tubes, hand-welded at great expense. This is a sizable fraction of the total cost of a Falcon 9 launch. The H-1s were cheaper individually (~\$400K) and would be amortized over several launches in your scheme, but they would still be a significant cost. More modern construction techniques could reduce the cost of the engines, but with a great increase in development budget and schedule.
SpaceX, making the engines and both stages of a rocket that they manage the launch campaign for, has a huge cost advantage here.