While the HT4100 is no power house, it is able to easily keep up with traffic and can cruise easily at 70 mph+ speeds. There is something wrong here.
I would bet it is an ignition timing problem, if you have eliminated the possibility of a restricted exhaust. Make sure the distributor has been correctly installed. I know in a RWD car, this can be a bear to get at. Then be sure the timing has been correctly set as per the sevice manual. This should clear up the problem. Also, it has been found that by advancing the base timing a few degrees more than what is recommended can really wake up these engines with no ill effects.
If this does not clear it up, make make sure the cam timing is correct. This can be a big job to ascertain, but it may be necessary to clear up this possibility. It is suspect with a freshly rebuilt engine.
You can also check for DTCs to see if any exist. If so, diagnose for and repair. Perhaps this could be part of the problem. Over or under fuelling caused by a defective sensor can cause a lack of power and adversly affect fuel economy. Verify a good ECM as well.
I would recommend obtaining the correct factory service manual as well. Such symptoms are outlined in section 6E.