can I upgrade the heater core in my 87( no A/C )to something bigger and maybe a better fan, mine doesn't throw alot of air either.

 

You'll have a tough time finding another core that fits in the box. Look at your doors and seals around the core. You might find that you have the defrost or floor door stuck open and that's why you don't get much air out of the vents.

Another trick would be use a high temp thermostat, like a 195 or 210.

 

I would agree that you should double check the current condition of the doors/seals to make sure that they are intact and in place. If they aren't then you would obviously have issues as to the amount of air flow. Also, make sure that the fan is running at the proper speeds and isn't being blocked/hindered by anything that might have gotten sucked up into it.