[406 Q-ship]

If that is an LS-6, it is one of the last run service replacement engines. It has a high rise square port intake on it, it doesn't mean the heads match but by the way you describe how it runs I believe that the heads are square ports too. It is a really late replacement service engine because GM dropped the low rise "corvette" intake in the very late 1980's and went to the intake used on the L-78/L-72 intake (high rise). I bought one of the last run LS-6 service engine in 1989 and it had a high rise L-78 style intake mainfold in it. Casting numbers on the service engine replacements can get weird and will not necessarily match old casting numbers. Check the front pad for the engine code would be the best option but not an end all. My LS-6 did have the 7.4 cast into it like the later GM blocks.