Hiya..
I am going to assume that the Dart does not have a Big Block in it at this time.
FWIW, I took our 1968 Dart GT (original 273 V8 - ) and retrofitted a 383 in it.
I did a lot of shopping around for all parts...from the 8 3/4 rear end, to the pumpkin to the Transmission and Radiator...
I spent $12K.
All work, except paint, was done in my garage. I'm talking from welding home-made Frame Stiffeners to assembling the engine to retrofitting disc brakes from a '73 Duster with a Mustang Proportioning/ Metering valve..
$12 to $15 K is a reasonable budget..
IMHO, The engine is Not the biggest money pit...It's a major part, but for me, it was only about 1/4th of the money on the build..
Other things add up real quick. When you go with a Big Block Retrofit, You gotta upgrade just about everything, from the radiator to the brakes to the suspension (Torsion bars & Shocks) to the rear end to the wheel-wells..
Also apropriate transmission (a small Block 727 auto trans will NOT work on a BB engine) You'll need a BB 727, Torque converter and an A-Body 8 3/4 Rear end.
My best advice to you is to buy one piece at a time and to treat each part of the retrofit as a separate little "project". Then, set up a Schedule for completing each "project".
Headers - One Project
Carburator & Linkage - One Project
Frame Stiffeners - One Project
Radiator/cooling fans - One Project
Also don't stack up more than 3 - 4 "projects" at a time... or you might get overwhelmed and/or lose interest.
This system worked for me. Our car was my fist build of this type, so I was completely inexperienced when we started.
Stick to your schedule and you will be OK.
Good luck to ya.
--