The nine-year-old was a well-supported sixth behind Noble Yeats in the Aintree marathon last year and is currently a general 16-1 chance for the four-and-a-quarter-mile spectacular on April 15.
He reverted to hurdles on his first three runs this season, including when scoring at Limerick on Boxing Day.
However, he unseated JJ Slevin at the first on his return to fences in the Bobbyjo Chase at Fairyhouse last month.
Martin Brassil, who enoyed Grand National success with Numbersixvalverde in 2006, said: "He will probably run in a three-and-three-quarter-mile chase at Down Royal on St Patrick's Day. That will be his last run before Aintree. I've been happy with him.
"I'm reasonably happy with his National weight at 154. As long as the weights don't go up, he has a grand racing weight.
"He ran a nice race last year, but probably over-raced a bit. He probably got a bit exuberant. You have to remember it is a long way round there twice."
The Kildare handler is still no nearer to deciding which race Fastorslow, who is similarly owned by Sean and Bernardine Mulryan, will contest at Cheltenham next week.
The seven-year-old, who was runner-up in the Coral Cup at the Festival last March, holds entries in both the Ultima Handicap Chase on Tuesday and the Magners Plate Handicap Chase on Thursday.
Fastorslow has run just twice over fences in Ireland, both coming in Grade One company this season.
He finished fifth of six to Galopin Des Champs in the John Durkan Memorial at Punchestown and last of five to Gentlemen De Mee in the Dublin Chase at Leopardstown.
Brassil lamented the lack of options for Fastorslow, who won his only other chase outing when scoring for Arnaud Chaille-Chaille at Auteuil in September 2019.
"The conditions of a lot of the races over here excluded him from having a run, because he won in 2019 and the conditions said you couldn't have won before May 2021, and that is why I had to run him in a couple of Grade Ones," he said.
"They have given him plenty of weight, too, especially for a horse who hasn't won a hurdle race since he was three.
"He won a chase when he was three – now he's seven and rated 150 over fences and 147 over hurdles.
"At the moment we will go to the Ultima, but we will confirm him for the Plate as well. The weights have already gone up 4lb in the Ultima, but we'll wait and see."
He added: "He is hugely inexperienced, even though he does jump very well and we wouldn't have any worries about that. At the same time, it is a big ask. If he has a bit of class, it will have to come out."
Brassil will run An Epic Song in either the Coral Cup or the Martin Pipe Conditional Jockeys' Hurdle next week, although he would welcome slower conditions for the six-year-old.
"He would be very much ground dependent," Brassil added. "He would want a real nasty turn of weather.
"I confirmed him for the Coral Cup. If he doesn't make the cut there, he'll go in the Martin Pipe.
"He'd want a lot of rain and about a foot of snow to melt, like at the old-fashioned Cheltenham – the Desert Orchid days or days of The Thinker."
