All 30 SFL members met at Hampden on Thursday to examine proposals for a merged set-up also involving the 12 Clydesdale Bank Premier League sides.
A 12-12-18 structure has been proposed by the top-flight body, and the lower-league organisation has now vowed to join them in examining those blueprints.
However, a revised set-up which dispenses with the 18-team bottom division and instead uses two leagues of 10 to offer a four-league solution of 12-12-10-10 will also be looked at before a final decision is made.
Longmuir admits all that will take time but insists the new league could be in place for next season if the clubs demand an early start. For now, though, he accepts progress must be slow while talks continue.
He told Press Association Sport: "This has been the first time that the SFL have had a chance to get together as a group of 30 since we started the dialogue on league reconstruction. You have got to give it some context in that respect.
"I'm pleased at the way the meeting ended up. We had very, very strong support to move forward to the next stage in the discussions with our colleagues in the SPL and the Scottish Football Association.
"We are now going to start looking at the real detail of rule books and other areas you would expect when you are trying to combine two organisations. Nothing, though, has changed from an SFL perspective other than we have had an opportunity today to have everybody from the SFL together - clubs talking to clubs, clubs listening to clubs.
"There was a distinct mood that we are still keen on a 42-club solution to take the game forward and that is what we got. We were talking principles today - one league body, governance, distribution - and those were the things that were key today.
"Now we have had a clear message that our members want to take forward the full detail and we will now look at a combined rulebook. That will not happen overnight. We will have to work on that and at some point we will have to bring it back in a more formal package to the 30 clubs and allow them to make a much more informed decision."