273 Mystery Motor

So my strategy: It seems both the 2BBL and 4BBL pistons can handle the Isky E-4 cam. If I replace the current cam with E-4 and appropriate solid lifters, pushrods and adjustable rockers I should be good to go, correct?

I just have to be careful to use the 66 pushrods due to the block stamping of B273 11-19, correct?

Geez Ma Mopar didn't make it easy with this one.
They actually made it very easy. People make it difficult by not knowing what they have. You will save yourself a LOT of time, trouble and money if you buy a factory service manual and learn to do this yourself. That way, you'll know what you have.