If I wanted to do this swap what would I need to look for? Seems the 74-76 models are a little different tan the ones before and after. Not sure if  what year cylinder and head I would need.

"75--75--76 125 & 175 use the same 1WE base gasket so you'd be safe swapping any of those years i'd think.. Also "73 CT3 uses the same one so chances are---!! Unless rod is a different length & piston pin set at a different height--but very much doubt it'd be different. That's my guess for the night. Reminds me i have a TY175 top end on my AT1 but think i had to make a base plate as there was 5mm difference somewhere--think was just the gudgeon pin height in the TY piston as in my case i was going from piston port AT1 to reed TY175. Goes like a rocket too.

74 to 76 DT175 will all work.  Get a head to match cylinder.  Only thing to know is 76 was first year oil was injected into carb so no injection port on cylinder base anymore.  With a 76 you'll have to add a fitting to carb.

And yes, you can also use 72-73 CT cylinder/head.  ...(or even 69-71 CT but those are piston port and not reed valve.)  

74-75 seem to be the most preferred years and I think they are exactly the same parts those two years.

No rod length changes to worry about.  You do need the 175 piston pin and bearing as they are longer than the 125 versions.  Bearing is a little tricky to buy the right one as parts lists show both and don't really say which is which.  Easy way to get right one is look up small end bearing for 77 IT175. 

After 76, with introduction of the monoshock models, Yamaha changed design and got away from the long through studs that hold head to cylinder...  so you need to stick with top end off an earlier model. 

Thanks for the info guys. Was there a difference in port timing and cylinder head between the 175 MX and DT or was the difference all in the expansion chamber and fueling?

Assuming you mean MX as in the USA "motocross" models, port timing is significantly different.  Early YZ125 cylinders will fit too.  YZ175 cylinder will NOT fit.  Not that you'd ever find a YZ175 cylinder anyway. 

Does anyone know where to find pistons and rings for the 74-76 175's seems i can find the 73 and earlier and 78 and up but noting in between

Check with Enduronut he has a good supply of pistons and rings...  or there are other options like a slightly modified Yamaha Blaster piston. 

The earlier (72-73) or later pistons should work too.  Enduronut is the expert. 

Just stay away from cheap aftermarket (Chinese) pistons.