It would be an interesting fight and it's one I've been wanting to see for a long time. Both are physically strong with solid BJJ games and can place you in all sorts of trouble on the mat. Spivak even in his slimmer form strikes me as more of a natural HW, but I don't know that that matters all that much. He is very much a committed judoka with mediocre wrestling whereas Jailton is a guy who has exceptional wrestling and has been tossed around by Judo throws before. IMO Almeida is faster and more athletic to a pretty damning degree... but if for some reason l the grappling got nullified in the fight, I would fully expect Spivak to box him up. Sergey has developed some pretty serviceable hands in these past couple of years with an emphasis on his jab. He even caught Ciryl with it a few times. Conversely, Almeida's capabilities in any protracted striking affair is still a big old ??????? as wild as that is to say about a ranked dude who's had almost 10 fights under the UFC's brand.
My initial suspicion is that Almeida would be able to use his speed to make Spivak uncomfortable by utilizing a bunch of lower-body shots in order to get in on his hips and topple the big man and force him into constant scrambles. Spivak would probably be able to explode out of a few bad positions and survive for a while, but I think eventually Jailton would either take his back and choke him or pound him out.
That said, I wouldn't be surprised to see Spivak beat him. Aside from the aforementioned striking advantage, I think the uchi mata/harai goshi could be there as counters to Jailton's shots, especially against the cage. We've seen other fighters hit those, including against Almeida. Spivak also has a nice front headlock sequence and heavy GnP. Jailton has shown some questionable fight IQ & killer instinct during his transitions at times, too.