Clean as much of the nasty stuff out as you can, with a rag and some solvent.
No sense loading up your oil and the engine with that stuff.
Shoulda cleaned it before you put the engine in. Lots of solvent and a
brush, then be sure and drain it out of the oil pan. If you have weeds along
your fence line that is a good place to use the dirty solvent.....
(it biodegrades in as little as a couple years in areas with normal rainfall.)

As a matter of curiosity did you check the valve lift before you installed
this engine? They are notorious for flat cams.
(In general an engine you know is always better than one you don't know!)

Did you install the distributor correctly?
Firing order is 1342 and you DO count from the front. However it is a VERY
common error to put the distributor (or wires) in with it firing on 4
rather than 1 (both are TDC at the same time). If that is the case just
move the spark plug wires straight across from where they are on the cap.

My experience with the 2-barrel downdraft carb has been uniformly bad.
SUs are a lot easier to deal with if they are in reasonably good condition.

If the cap fits the distributor it will probably work OK.
--
George Downs, Bartlesville, Oklahoma, Central US