OK i have a 1973 dart swinger with a 225 slant six and its auto trans . im going put a new rear end in it and i want to make it manual trans also. and last i will make the engine better well with a little help from here not sure what to add to it. OK so i need to know how to do any of this im new and not good at this stuff but want to learn. what rear end and gears do i need and what trans do i need that will work with the 225 and what mods are there to make the engine come alive. and is seeking 300 hp to much??

First off welcome to the board 1973dartswingerdms.
Before any logical suggestions can be put forth you have to narrow the selection by deciding what the car’s roll is to be. Are you looking for a full blown race car, daily driver, or something in between? Unless you plan on some sort of forced induction 300 hp is mostly pie in the sky except for an uppity racing engine build.

There are several builds posted here from economical perky torque motors to the high revving normally aspirated side of the rubric. There are on line calculators that can help one sort out drivetrain combinations, and engine builds. One rule pertaining to slants is; more power = more cost and that cost escalates real fast once the build goes beyond stock factory combinations such as the later model's use of Super Six.

So think about the way you intend to use this car, your budget including up grading suspension and brakes to safely handle the new higher power out-put and modern traffic, then get back to us with your ideas, and we will gladly try to help out.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC