Having a problem

My son has a stock 74 duster with the 225 in it. We lost oil pressure on the highway a few days ago and assumed oil pump because the car had been sitting for a number of years before my son bought it and at 37 years of age and 137000 miles I thought it was good idea to change it anyway. When we drained the oild in prep for the pump change we noticed a very strong smell of gasoline from the oil so I said AHA! fuel pump leaked in to the block and thinned the oil. We changed fuel pump and put the nice new higher volume (supposedly 15% more) oil pump on. On the first start up the oil light when off after about 3 seconds (God I hate idiot lights) and idled smooth no untoward sounds from the motor or anything else. Now though there is a noticeable loss of power at all times and a very slight surging when driving at 65 mph. I don't know whats causing this now and would like all of your opinions as to what may be causing this and how to fix it.

Thank you

Szlachta