OK Corral, Beware The Bear, On The Blind Side and Brave Eagle are all on course to run in the showpiece event Henderson last won with Triolo D'Alene in 2013.
Three of the four were in action at Newbury on Tuesday morning, with former champion jockey Sir Anthony McCoy taking the reins on ante-post favourite OK Corral, who was pulled up in the National Hunt Chase on his last outing.
Henderson said: "That will have blown away the cobwebs for OK Corral and he seems in good form.
"He was very impressive at Warwick (in January). The way he travelled through Warwick it didn't look like he needed four miles, but Derek O'Connor come over to ride him in the Warwick race to get him ready for the National Hunt Chase.
"We were tempted to bring him back to the RSA after watching that, but it seemed a bit unfair on Derek to do that so we stuck to the four-miler.
"What went wrong was in the interim period, when the flu outbreak came along and we were forced to revaccinate and it affected four or five horses and him particularly badly. We got him back in time, but he ran no race at all.
"AP said he finished his gallop very strongly. He was only getting going if you like. The other two horses weren't slow. I thought he had the right profile for the race, although I wasn't sure to start with."
Beware The Bear pulled up in the bet365 Gold Cup when last seen, but prior to that won the Ultima at Cheltenham.
"Beware The Bear looks in very good form. He has got the profile for it. He won two very good races at Cheltenham last season and he was fourth in the Ladbrokes Trophy last season," said Henderson.
"On The Blind Side ran a good race at Ascot (behind Vinndication). He has just got a little bit more to learn. He didn't need to come here as he has a run under his belt.
"Brave Eagle is not very big, but very game. He runs his races and stays. He worked well there with a two-miler in Pentland Hills.
"The handicapper has forced him into races like this now as he is in the 150s so you haven't got a lot of options. He would want better ground as he is better on that."
Henderson added of Triumph Hurdle winner Pentland Hills: "He is on the route for the International (at Cheltenham next month). It might be the right race for Brain Power, who won the race last year and has been on his travels and back."
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