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One. the acell plugs are designed for headers, and are called short header plugs.  It appears you have angled plugs.  If so many of the headers are not designed for angle plugs, so the PO might have used the short plug to remedy the angle plug problem. 

Know way of knowing unless you know the part number of the headers to verify where they will work well with angle plug heads.

If you have an aftermarket distributor, you will need to find the part number and get an accel cap and rotor that matches the distributor.

Just a word of advice, do not assume that any Department store computer will be correct when it tells you the part numbers.  In most cases I have seen they are wrong.  Wrong, mainly because over the last 30 or so years our cars have been modified or replaced with something other then original parts.

You are correct that you should have taken an old spark plug and your cap and rotor with you to try and match up the parts.  While Walmart might be the cheap place to shop, your better off spending a little more and shopping at a good speedshop or a major auto parts store when it comes to our cars.

Here is something that everyone should do, but we tend to over look and that is to create a log book for our cars.  We have one in everyone of our project and engineering cars at holley.  The book contains a list of what has been modified and the part numbers to those parts, and what the part numbers are for replacement parts like rotors, caps, plugs, plug wires, etc.  It also contains a list of what products to use on the engine, like thermostat temperature, oil type, oil weight, coolant type, transfluid brand, transfluid type, brake fluid brand, brake fluid type, etc.  Then there is another sheet that keeps track of all repairs, tests, changes, and a column of anything that needs to be repaired or change.  Its also good to have if you ever decide to sell your car or if anyone else drives it, plus it saves you a lot of time at the parts store looking up parts all over again. 

because of the headers you might have to go under the car with a box end wrench to break the plugs loose and the use your fingers to get the plugs out, After Shark is right about the shorter plugs made for headers you might want to stick withe the same, BB's have angle plugs,the new plugs you have look like a hotter heat range than the originals, which make them longer, in 74 GM went to te HEI type distributor with a larger cap and rotor with the coil built into the cap, thats what the box on top of the cap is for, guessing they gave you one for a 74, by looking at the photo it looks like someone put an earlier point type dist. in with a external coil, can't tell if it's GM or after market for sure, look for "Delco Remy" somewhere on the dist., also looks like the plug end of the wire's were arching on the headers, might want to buy a set of wire's with heat resistent boots, might also help to bang in a little clearance with a small ballpeen hammer where the plug ends are close to the headers, at least 1/4" between the boots and header.anips38505.9474421296

The length of the external insulator is not the critical part.  What is most important is the heat range, resistance, tip, gap, and all interior components.

But there may be slight differences in the plugs.  Even if both are acceptable, the difference could create a inbalance in the engine operation.  I see no advantage to useing some of each.  I would stick with all one type.

Plugs do in fact make a huge difference.  Resistance, heat ranges, tip design, and many other plug factors all have an effect.  And even when the application book says it's the right one, it may not be.  There are huge differences between plug mfgs. The best in one application is the worst for another.