Dont take any of the chrome or fish eye lights off they are only for the 68
Do you want something that twists off the line and pulls g's ,then 440 is the way to go,I love 383 short stroke engines but in a grudge match 440.
Parts rebuild is easy on the wallet also.A 400 stroker is money to make a 451
472 b deck engine.
Rb engines easy to get bore out .040 good heads headers.
318 easy to build short stroke 1967-1970 is the best with 360 heads or 340 x or j heads are good.
Tranny 727 or see what new over drives you can find.
Rear end find a truck 9 1/4 3:55 tough 8 3/4 getting very expensive.
The bolt pattern on your car is the small left handed threads.
I had in my career 3-69 darts 1-gt 68 1-270 68 dart and converted the first ones to a 400 then to a 440.2 Dodge Dart Sports with sun roofs.1 70 Duster
Blew alot of trannys so build one heavy duty.
Dodge Trucks makes excellent parts donors
Your Dart will be nose heavy with a B-RB block.Dont cut that hood save it find one allready cut.