235 popping thru carborator under load.

[jeffbcn]

Sorry if this is a long read but I need some advice.
I have a 47 thriftmaster with a 235 from 1962 and heads from a 261 1955 thru 1957 according to the casting numbers.
I have no idea about prior service. I have had it less than a year and put about 2500 miles on it. A couple of weeks ago I noticed intermittent popping thru carborator. I just took it on a 700 mile round round trip and when I came back the popping was more pronounced. if I ease it up to 2000 rpm it sounds great. Any higher or under load it pops and loses power. I ran motor flush and changed the oil. I timed it, new Dragonfire hei distributor. Still pops.
Opened the valve cover and it was dry, even some surface rust on rocker arms.i ordered new oil pump.

My question is this, can I just replace the oil pump, do I have to rebuild the heads or is the whole engine toast?

Any suggestions are welcome.