After Spence beat Shawn Porter on Sept. 28, boxing fans have been hoping and praying Spence and Crawford can get in the ring for a showdown at 147 pounds.

Well, we spoke with Bud who tells us he's ALL THE WAY DOWN -- sending this message to the undefeated Spence:

"Take off them panties and pull your pants up, and let's sign the contract!!!"

Crawford says he's been trying to get the Spence fight for a while but feels like Errol's camp is always full of excuses. Terence says the real reason is Errol knows he'll lose.

"I honestly believe when it comes to him ... deep down in his heart, he knows what he's up against when he goes up against me."

And, get this, Crawford -- the reigning WBO welterweight champ -- says he's not convinced Spence deserved to get the split decision victory over Porter either.

In fact, Crawford thinks Porter should have WON that fight!!!

"It was a close fight. Don't get me wrong. I wouldn't say it was a big-time robbery. But, it was a close fight and if I had to give anybody the victory, I think Shawn deserved it."

32-year-old Crawford is widely considered one of the best pound-for-pound fighters in the world and 29-year-old Spence is also a Top 10 PFP fighter.

In other words, this would be a MEGAFIGHT.

One of the obstacles ... Crawford's promoter is Bob Arum. Spence is an Al Haymon fighter. And, Haymon and Arum don't typically make fights together.

But, they made it work for Mayweather vs. Pacquiao ... which means if the money's right, the guys will fight!!!

Now, everyone pray to the boxing gods ...