What would you do? Thread or remove.

I’ve done quite a few of these retrofits, most of them being converted to home run manifold distribution. Much of that depends on how many supplies and returns reach the basement ceiling vs. how many radiators there are. If the numbers don’t match, you can’t use 3/8” or possibly not even 1/2”. One pipe could be feeding more than one radiator. I’ve seen it many times. If the basement ceiling isn’t being finished, I’d do my best to leave a male or female thread somewhere and then just reduce it to 1/2” or 5/8”. The pipes/fittings that are buried on the on top, bottom, and both sides are tough. You have the pipe still extended out and cut. Is it cut square? If not, cut it square, de-bur it, and use a Ridgid 600 or 700 and thread it in place with a big wrench as a backup a foot down the pipe. Two man operation. If the adjacent wall is in the way, open it up. If it’s not load bearing, temporarily knock out stud and replace it when you’re done. If it is load bearing, do whatever is necessary to temporarily support it.
A Ridgid 700 and some screwed fittings are lot less expensive than a MegaPress and it’s fittings. We did a bunch of these in the Hyde Park neighborhood of Chicago. We only had issue. On one project. Two rooms on the third floor, shared the same supply and return for each radiator. Luckily it was fed by 5/8” pex but getting them somewhat balanced was a major pain in my arse.