Any clue on what temperature the '98 sunfire heater should be putting out? Just replaced the heater core and is ranging 130-145 degrees, old heater core sure seemed hotter.

The air from the heater core will be as hot as the coolant temperature of the engine. 80 Degrees is the opening temperature of the thermostat and that will be where it remains until the engine starts to work or the ambient temperature rises.
Thermostats come in a couple of different opening temperatures so check around for one that opens around 85 degrees if you want hotter air in the cab
Another point is to close the recirculating vent door to the cabin as the heater core is trying to heat fresh freezing air if it is open

The heater core may be plugged with rusty mud. The blend door may not be moving to heat position. You can tell if the heater core is plugged by putting your hand on both heater hoses near the firewall after the engine is up to temperature and the heater controls are on and temperature is at 80 degrees. Both hoses should feel the same temperature. If one is way cooler than the other, your heater core is either plugged or the control valve is not letting water into the heater.

It should fall between 200 and 250 degrees F, I take it when you say 90, that means 90 degrees Celsius, that equates to 194 degrees F.

You are within range, actually running little bit cooler, which is GREAT!

by flushing the rad, you probably plugged the heater core up. Good to flush the whole system

Heater theromstat is faulty, it is located near the heater core in most vehicles, It monitors the temperature of the core or in some vehicles the inside cabin temperature and open or closes allowing heated water to flow through your heater core. Look on your heater hoses and you will find it. Most of the time its a DIY job and not that expensive but don't bet on it being cheap, they are proud of some of those things. Just have to check with your local parts house and find out. Used to be a common problem but I guess technology has caught up with them. Hope this helps.
Gillight

Remove or Disconnect

1. Negative battery cable.
2. Drain cooling system.
3. Heater hoses from heater core.
4. Instrument Panel.
5. Heater core outlet.
6. Heater core cover.

• There is a mounting screw located in a recess in the center of the cover

1. Heater core mounting clamps and beater core.

Install or Connect

1. Heater core and heater core mounting clamps.

Tighten

• Screws to 1 N.m (9 lb. in.) .

1. Heater core cover.

Tighten

• Screws to 1 N.m (9 lb. in.) .

1. Heater core outlet.

Tighten

• Screws to 1 N.m (9 lb. in.) .

1. Instrument Panel.
2. Heater hoses to heater core.
3. Fill cooling system and check for leaks.
4. Negative battery cable.

Tighten

• Bolt to 16 N.m (12 lb. ft.) .
• 
  

Here's the way I usually approach problems such as this. First I'll hook up a scan tool that allows me to see the actual engine coolant temperature is [not just what the gauge says] as the vehicle is being driven down the road at highway speeds. The coolant should be near the 192 - 200 degree range to get the best heat out of the heater. If the coolant temp is below that level then the amount of heat you'll get will drop according. IF the coolant is reaching full temp but the air coming from the heater isn't very warm then you could have a restricted heater core or a temp control door in the ductwork that isn't working properly and is allowing cold air to sneak past the heater core. Usually if you have a water pump impeller problem you'll also have an overheating problem as the pump often isn't able to move enough water through the radiator to keep the engine cool. Hope this gives you a starting point and good luck!

First of all, make sure the heater core isn't air locked. With the engine running, and your heat temperature on high, loosen one end of the heater hose and see if you can get the air out of it.

are you sure your ac is working at all. a old school solution would be to put a cutoff going to the heater core in the water hose.

Since your blower is working and it is a heat issue,thats where we'll start,First thing is have you checked the coolant level in the radiator when engine is cold,make sure it is filled to top,next verify you have coolant in reserve ,Start your van and get it up to normal operation temperature,If your heat gauge on the dash works correctly it should be 190 degrees,Now lift the hood and locate the two heater hoses going into the heater assembly at the fire wall,Both hoses should be hot to hold on to,if both are hot and no heat inside your problem will be the blend air door in heater assembly.
if neither hose is that hot you need to replace the thermostat in the engine to get it up to 195 degrees.
if one hose is hot and one is luke warm then the heater core is blocked and to blame. hope this helps you.