Well here is are a couple things to think about... IMO....if you are thinking of twins...
1. build the transmission.....
2. consider at least getting 30 to 40 over injectors
3. Lift pump..
4. If you have EFI coming....no sense in paying for tunes twice... because if you buy tunes then get twins, you are paying all over again.
All together a rough ballpark figure for twins..I'm talking parts only... 3500-4000 ish.... with injectors add another 800 or so....tuning another 500.... then labor for all if you are not doing it yourself.
Tranny build...you are looking around 4000-4500 depending who you go to and what kit you get. I would def. 100% build the tranny first.. Me personally, I would do the tranny, lift pump, and EFI live first...then see if you even want twins.