i did a full removal and complete strip down of the heater in a 1971 Traveller, as the heater was pathetic
you can do little better than to see this great video...
https://www.youtube.com/watch?v=v75LjUrVaPE
i installed an 88 deg thermostat ...having found there wasn't one fitted at all !!
i disassembled the whole unit, flushed through, but decided to refit with the larger heather matrix and hoses ...new foams where needed &
resprayed the tin work ....i only wanted to do this job the once !
in reassembly I adhesive sealed the heater body end caps, and sealed off any spare holes so no escape for that wonderful hot air
flushed out the engine and rad as well, on the basis of removing ALL crud as a new heater was going back in...why re-contaminate it ?
a fiddly job, but the end result was well worth it ...the joy of
hot air to the car and a respectable, if modest, flow to the screen that does actually defrost / de-mist pretty swiftly