I have a 73 Dart Sport that was a 340 car. I am going to put a big block in it using a motor plate. I this K-frame worth removing and using any a-body k-frame like out of a 6-cyl or is the V-8 k-frame stronger? This is going to be a strictly strip only. Also - I thought 73 was the transition year to tegh 4 1/2 bolt pattern - where 4" BP on the drum cars and 4.5 on the Disk. I must thougth worng as this car has 4.5" BP with drums. - How hard is this to change to disk? will I need new spindles?

Thanks!
Jim

The 73-76 A body cars had spool type k members that most enthusiasts agree is superior to the biscuit/pad type. Personally, I'd pull and sell the original and slip in a common 68-72 unit since you plan to use the motor plate.
The brake situation with A bodies is a bit fuzzy, but here is what I have seen in years of 'junkyarding:
67-72 were all small bolt pattern. Most were 9 inch drums on all corners.
73-76 used small bolt pattern when the cars had 4 wheel drums and the larger 4.5 bolt pattern when they had front discs. YES, some cars as late as 1976 did come with front drum brakes with the small pattern. I've seen them and I have owned one: 1976 Dart Swinger 2 door with a slant six.
Here is where your car fits in: The large pattern drums up front probably are from a 62-72 B body since they used a spindle the same size as the disc equipped 73-76 A bodies and they shared the same sized upper ball joint. It is easy to swap a drum assembly in there if the discs were pirated for another project.

I have heard rumors there were some BBP drum brake cars, but I have never seen one myself.

BBP drum A-body cars are most likely swaps from a B-body. There isn't a part number for a BBP drum brake A-body setup in the parts book.