Hello, Tiger, and welcome:
Not sure what "build it up" means. What parts do you have?
The Spencer Oft Requested Info ("SORI") section on this SSS Forum may help on barrel dim's.
If you are going to start with just a receiver, gathering the other original parts will be costly, time consuming and can involve parts that will not fit! Original parts dimensions vary and I have seen some frustrating situations.
One cheaper, time saving alternative would be to contact David Stavlo at Lodgewood Guns in Wisconsin and discuss using the receiver as trading material toward a complete Spencer. He often has them. Good guy+++.
If you need only a barrel, try Larry Romano at Romano Rifle Company in Pennelleville, NY. He makes VERY HIGH QUALITY finished and impeccably chambered Spencer 1860 and 1865 barrels in both multi-groove and three L&G configurations.
How about posting some pic's of your 1865 receiver, please? Thanks.
Respectfully,
El Supremo/Kevin Tinny