I currently have a stock 75 dart swinger and am building it primarily as a driver/highway cruiser and would like to swap in a lockup 904 and an 8 1/4 rear from say a diplomat, are there any issues with this swap? Thanks.

The trans wouldn't be a problem(as long as its a slant trans not a v-8 trans) but you would likely have clearance issues with the rear. An M-body rear is wider than an A-body rear. I put an F-body rear (I think it is the same width as an M-body) in my 70 Valiant years ago. It worked but I had to use stock width wheels and the tires were still very close to the wheel wells. They only rubbed once over a large bump, with an 8 3/4 in the trunk, and tires aired down to 20 psi for racing.

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Thanks for the quick response and info. I think I answered my own question by reading this http://www.mymopar.com/index.php?pid=73 and guess I just need to find an 8 1/4 from a 73-76 abody.

What 70valiant' said. The trans should not be a problem, as long as its for a slant. An F-body rear is the same width as an M-body rear. I bolted in an 8 & 1/4 rear from an '83 Diplomat cop car into my '78 Volare.
An A-body rear is narrower. You might be able to make it work though, but I've never done it, so I can't say how much trouble it would be.

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"Ja, Ich fahre ein altes auto."
'78 Volare 225
'67 Charger 318