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Hmmm, another C-6? Any other trans will require a different driveshaft. Otherwise, a c-4 or FMX will work, but entail a different trans crossmember and driveshaft... C-6s are easy to rebuild...

Just for the sake of asking (in case I've forgotten...), why a 302 in there instead of the stock 351C? What year 302? Remember the flexplate balance issue with any 80s-newer 302/5.0...

Where are you again? (please put your location in your member ID/ profile info so we know where you are!) I'm parting out two '73s shortly, one with a FMX and the other with a C-6...
 

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Just to expand on what Milo said, there's no way to recommend a "best" choice without knowing your priorities.

For ease of replacement, another C6. (Although, as previously noted, its rare for a C6 to be damaged to the point of needing complete replacement. In the vast majority of cases it'll be more economical to rebuild.)
Best for performance would be a C4, since it'll be smaller, lighter and more efficient than the others. But the C4 is not as strong in stock form, and will require various linkage, driveshaft and related changes to replace a C6.
Best for originality would be an FMX, since that's likely what came in your '71 from the factory. But like the C4, its not a drop-in replacement.
Ultimately its a matter of personal preference and the trade-offs you're prepared to make.
 

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Discussion Starter #4
Im in Murfreesboro Tn right outside of Nashville. I had previously fixed a problem which I thought was my carburetor but turned out to be the distributor, shaking bad and running rough and just plain horrible. Replaced the oem ford distributor with hei gm style and fixed a lot of the problem. Although the shaking still occurs but not as bad, Realyy only happens when you put it into gear. Took it to a trusted mechanic and he said feels like planetary gears are bad. Since this is my daily driver I dont really have time to pull the trans and rebuild it, hence it would leave me without a ride. I do however have a old friend of my father who build racing transmissions who said he would go through it if I brought it to him and fix it for cost of parts. Sounds like the most logical idea. just dont know how long it would take him to get to it.

Remember the flexplate balance issue with any 80s-newer 302/5.0...
- What does this mean? The motor I have is a 84 5.0/302, would this explain the reason my car would shake? the unbalanced?
 

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Could very well explain it. Not really "unbalanced", but improperly balanced...The early cars used a 28 oz. balance, the later 5.0 use a 50 oz. imbalance.
 

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Remember the flexplate balance issue with any 80s-newer 302/5.0...
- What does this mean? The motor I have is a 84 5.0/302, would this explain the reason my car would shake? the unbalanced?
All 302s use weights on the flywheel/flexplate to balance the crankshaft. Up until '81 or so, the weight was 28 oz., after that it's 50 oz. Most, if not all, small block C6s came with a 28 oz. balanced flexplate. Using an original flexplate that hasn't been rebalanced to the later engine will cause a shake. But, that type of shake usually increases with rpm, so if it goes away when you rev the engine, its doubtful that's your problem.

PS: I see that Woodsnake beat me to it. Anyway, also keep in mind that there is a difference in firing order between late and early 302 engines, so be sure to verify that your distributor is wired properly, since the wrong firing order can definitely cause rough running.
 

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Discussion Starter #7
The problem occurs when a load is put on. You can really feel it almost vibrate/wobble/(guessing this is the same thing) when you hit about 45 and up. Almost as I stated a while back a rotating vibration. But basically if I wanted to, I could pull the block number and get the year and Im sure somewhere on the transmission is a stamped number that would indicate year. This would narrow down the problem of the imbalance?
28oz as opposed to 50oz seems like a very large difference to me, almost being half. I would imagine the problem would be very prominent. This is an ongoing issue I have been having trying to figure out what it is. Started as just the rotating vibration during load, but rpm doesnt seem to have that much of a effect, or so it seems. Slowly though it has gotten worse. To the point of a very prominent shake when the load is put on. I recently came across some money finally may be able to address the problem. I have continued to drive the car almost daily 6x a day on the interstate roughly 60-65 miles each day. Im guessing this overall could have done more bad than good. Ive had and been driving the car since September of last year.
 

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Discussion Starter #8
Also looking online the flexplates I find for a 302 usually say c4. Out of the ones I have found, none of them said c6. Typically say 164tooth 50oz. My question is c4 and c6 plates the same?
 
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