If you chose to retain the 1000 radiator, then give it to a reputable radiator shop & ask them to pop the tanks off & have a good look at the core.

Most 1000 radiators have either had a replacement core by now, or are due for one. I have counted up the tube numbers & fin-per-inch count in a few of these & they vary a bit, but if you are up for a replacement core, then tell them you want the heaviest duty core that they can supply. The extra cost is a mere pittance, but the added security is worth it's weight in gold.

I had a new core made for mine with more tubes & fins, but sadly, there wasn't quite enough room for & third row of tubes. Never mind, I'm happy with it.

I also recomend a cheezecloth condom type of filter in the top tank fitting. Your new engine will undoubtably dislodge particles of gunk from the block & your nice clean radiator will filter out the lumps so that they don't end up in the block again.
A coolant filter will trap them before they end up in the radiator. I was amazed at how much gunk was in my filter about 5,000 miles after I had given the engine in my 1200 coupe a thorough flush out.