Black paint on the exhaust manifold will help it radiate MORE heat. Remember
your physics class - blackbody radiation??
You DO need a heatshield or you will boil the gas in your carbs and get
vaporlock which will absolutely keep the engine from running at all.

How far is your ignition advanced? Could very well be retarded ignition which
kills torque faster and faster as RPM increases.

Did you measure valve lift? Over 3/8" at every valve? Shoulda done that
BEFORE installing the engine when it would have been EASY to change the cam,
which is also one of the "usual suspects". Also if you had a D or K cam
before and have a C cam now the response will not be as good.

It is a LOT easier to do your homework "before class" and also (I found out
AFTER a very difficult time graduating from Stanford) a much better way to
learn than through panic attacks.

Your felt seals are on the way.

They would not have put bigger valves and ports in the E and F heads if
they didn't help. They work toward satisfied conventional drivers, not
potential hot rodders.

If the engine was dirty and you put clean oil in it, OF COURSE the oil gets
dirty! Note Murphy's law of cleanliness:
"In order to get something clean, you must get something dirty."
Corollary #1: "You can get everything dirty without getting anything clean."

Until you get it running right, don't go spending your money replacing things
that are now working. If your distributor cap is not marked internally,
it is OK, rotor probably also. Colder plugs are such a marginal improvement
(and NOT the cause of the problems you are facing now) that they should also
be put off until you know where you stand with the engine in general. In other
words, make sure your cam is working and that your engine is properly tuned
and running right before you confuse the issue by changing a lot of other
things.
--
George Downs, Bartlesville, Oklahoma, Central US