First you have to find out if it is a Early (Real) Spencer made shotgun or later made Bannerman made one. Bannerman ones should be marked on the side of the receiver with their logo . The early Spencer made ones bring good $ to a Spencer collector. The last one I bought was $350 and that was a very nice Bannerman made one. A nice "Spencer" will bring more. If it is a Spencer it is worth more than you payed as parts.
If ya want advice,the project sounds interesting but ya may blow yourself up. the receiver,bbl etc were for black powder only! My advice ,sell it to a Spencer collector and buy a new shotgun with the money OR start collecting Spencers! I have collected Spencers starting in 1967 ,have 26 rifles and 3 of the shotguns.The book ya need is "The Spencer Repeating Firearms" By Marcot for info. and the New Flaydermans price guide for value.
But the Spencer book cost about what ya payed for the gun if ya can find one,,no one ever said this was a cheap hobby. Getting off track I told my bride after transfering a nice M1 Thompson ,I m going back to collecting Corvettes it's cheaper.HAHA Have fun!