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One of the Corvette magazines, I can't recall which, was swapping engines in an early C4 with an LS1. If I'm not mistaken wherever an LS1 will fit, so will the LS6. They did mention a problem with motor mounts not "matching" and the LS1 was a little longer flywheel to timing cover. I'm sorry I can't recall which mag and issue.
There was someone here who posted about putting an LS1 in a C4 (an '84 if I recall correctly). Since all the external dimensions are the same as an LS6, it would be the same. Last I saw, he was almost done. He had worked through all of the mounting issues. Of course, I can't remember his name, but he posted in this section and also the engine mods section.
You might try the search function to see what you come up with.
TracerX, who is a member of the forum has an LS1 in his 84 Vette. The engine is in and has been running. It has been a couple of weeks since I was by the shop, but it was back together, just needed a few lines hooked up and would be back in business. Shop is in Houston, same shop that did my engine
If I was doing a gen III swap I wouldn't mess with looking for an LS6 as they wouldn't be worth the extra dough that the seller would want and the extra effort it took to find one. I would consider the 6.0L out of the HD trucks and the SUVs, the early ones had iron heads which you don't want, but I would only consider them if they cost that much less and I was on a budget. The aluminum headed ones would be lighter than your stock motor however. Then the LS1 from the Vettes and ponycars would be all aluminum and offer the weight advantage. If you can, go for the latest ones (2001 up) because of the improved intake, but if they cost more than $500 extra don't do it as thats how much the newer intakes cost. Save the extra money you would spend on an LS6 and get your heads ported and a new cam. If you getting into modding it the part you would have the most difficulty with is getting tube headers, because if the C5 versions don't find, custom headers are your only option(read $$$$$$). The gen IIIs feature 15 degree heads and spread ports, both of which are very different from any motor that came in a C4. Just my thoughts because I have been contemplating the same thing. Good luck!
And then, there is the tranny, which is a whole nother can of beans. You would need to stick to Fbody or truck trannies as the C5 ones are in the back. I imagine an automatic wouldn't be so difficult, perhaps they used the same ones as C4s, but I don't know to much about them because they don't really float my boat. If you are emphasizing strip duty they would be better however. But if its a manual your looking at I would have to say that would be a little harder. I swapped a '95 ZF6 into my '84 and it was pain, when the ZF came from a car that was essentially the same. But if you are going for the manual T56 I know atleast one member, Red5, could help ya out as he has put one in his '89 in think it is. He could help ya with clearance and clutch pedal linkage issues, you shouldn't have to worry about the flywheel, clutch, and starter problems people like Red5 and I had wouldv'e had.
From: Pottsville, PA. USA Home Of America's Oldest Brewery Yuengling
Re: LS6 into a C4? (lowflyer)
At GM's @Carlisle there was a 64 Corvette convertible with a ZO6 engine in it. The company that built the car is The Nicholson Company 408 Wirt Avenue Hanover, Pa 17331 717-637-7115.
This company specializes in GenIII engine conversions LS1-LS6-LS7-LS8. They had a 63 Chevy Impala with the 6.0 truck engine in it. The 64 Vette look stock until you looked in the engine bay and there was the ZO6 in it. They made the stock bellhousing and muncy trans work with it. :cheers: :chevy