69 Grand Prix heater core replacement?

Hi, replacing a leaking heater core with a new one in my 69 Grand Prix. I got the old one out but having trouble getting the new one in. There is a retaining clip inside the housing by the hose attachments which won't let the core slip in. You can't get to from the inside and you can't remove it as it needs to hold the core in place for when you attache the hoses. Is there a trick to getting the core to slip in?  

is there a screw from the outside of the case holding the clip in place... it has been many many years since i had one apart...

i seem to recall having a screw holding that retainer...

one more thing..??????

is the new core.. the same size as the old one...   i have run into the problem of the sizes creeping up and people stuffing them in the boxes and shipping them out for sale...

an eighth of an inch makes a difference sometimes...

be sure that you test it for leaks before you put it in.. if you have struggled with it...   a hand vacuum pump to 7 inches of vacuum...  you don't want to crush it..

i might look in my old manuals... if you post if your car came with AC i would imagine that most did...

as for size drifting...  don't forget ... the people putting the heater cores together today are young enough that their grand parents could not have driven one new..  so its just a size... from a photocopy...  of a photocopy of a photocopy... of a faded original.. that is long gone...

there are variations in sizes between brands ..... sometimes.....

don't throw out the original... there are some radiator manufacturers that can recore them...  with the exact size and using your parts as samples...  

you might also want to check the freeze plugs... the side ones near the back of the block....   if there is any leakage at all...  knock them in and remove them... then flush the block out... there is a lot of sediment buildup in the back of blocks after that many years...    this sediment is part of the stuff that can re clog the heater core..