Hi there John k, just in case it may help, over the years, i have replaced older heater cores,
mainly because of leakage,and require repair or replace, i have never did this to a Pagoda SL,
but a good source for a lot of old classics is GM, Cadillac, Buick,mid 80s to 90s, yes i know its not stock, but alot of
heater cores are hard to replace because of access,especially if the car has A\C, so it is good to get
a heater core a little smaller in size ,still solid copper,then shape or form the inlet and outlets
to the size required and use sealer foam around the core to support it, always easyer to replace later.
The photo is a evaporater from a Porsche 911, but gives you the idea.