I would stay with the 15#, as the heater core will not like 19# at 50+ years old.
The oil pressure does sound like you got the high volume/pressure pump. Contact Melling and ask about converting to standard pressure. Maybe they can send you a spring... ?

I would like to know, is the trans cooler attached to the radiator ?

Nope. I am still using the radiator trans cooler. I have the external cooler plumbed in series.