I used the XR700 on one or both of my conversions. It worked fine for me.

As for plugging the vacuum line, normally there would be a line from a ported vacuum source which may or may not have vacuum on it at idle. You pull it at the distributor to remove any advance it may be inducing and plug the hose to eliminate the vacuum leak created by pulling it off. It may be that a PO did away with vacuum advance altogether.

In the bowl of the distributor, under the points/pick up, there are some weights and springs. This is the mechanical advance. As engine speed increases, the spark needs to happen earlier. Centrifugal force pulls the weights out which causes the part of the distributor shaft with the lobes on it to rotate. That changes the timing of the spark. By running the engine at 4500 rpm, the mechanical with be “all in”. Setting the timing this way may leave you with the timing off at idle but should give you what the engine wants.

Because you have an optical pickup, you won’t have a dwell adjustment. By adjusting how open the points are at the tip of a lobe, you vary the ratio of open and close. It’s important to give the coil time to do its thing, dwell is this time. I don’t remember if the XR700 changed the dwell based on rpm, some aftermarket ignitions do, some don’t.

Michael