Heatercore & hose barbs
 

It looks like a heatercore does not come with hose barbs installed (from pictures on heatercore4u.com). Is it OK to slip the hoses over the core's inlet/outlet tubes, or will this result in leaking (even with hose clamps)? Where does one get those welded on, and would it just be more cost effective to get something like the Dtek heatercore radiator?

No, heatercores don't come with barbs. Will they leak when you use hoseclamps on hoses slipped over the heatercore? well, they don't leak that way in car setups with higher pressures and flow rates, do they? If you wanted barbs on there, soldering them is about all you'll be able to do unless it's a copper heatercore. If you have a copper heatercore and brass fittings you may be able to braze it. Personally, I recommend you go with the hoseclamp setup. good luck

Only problem is most heater cores come with 5/8" or 3/4" connections or a mix of the two...

True, so you'll need to stop by your local hardware store and pick up a few inches of 3/4 inch tube and a reducing fitting.

Hmm... reducing fitting sounds like it would produce a good amount of backpressure. Anyone using such a setup have any problems with it?

Also, think about this. Here's the order of your system:

pump -> waterblock -> radiator -> pump

If you had wider tubing between the waterblock and radiator, and even the pump and radiator, wouldn't that slow down the flow of water and improve cooling? I guess it would require either a reducing fitting or a wider barb on the waterblock exit. Hmmm. :P

btw, I see a Chevette heatercore for $17 at Autozone... I don't know if it's copper, though.

The reducing fittings wont add any more backpressure. Your not going any smaller than you normally would(3/8" or 1/2"). Your reducing the larger 3/4" down to your size...

You want to be doing Pump -> Radiator -> Block btw, no point cooling the water then heating it up with the pump before it gets to the waterblock.