Based upon the numbers that you posted, it looks as if you have a 1976 400 with 6X heads. X7 was an engine code used on the 400 in 1976 and 1978 (and probably some other years???) but since the head date code suggest 1976 then the rest of the engine probably is also.
The 6X-4 heads were used on 350's in 1976 and the 6X-8 heads were used on the 400 that year (both heads were used in subsequent years also). The date code on the heads (F146) suggest that the heads are from 1976. F=June, 14=14th, 6=1976.
The 0496947 is something like an intake manifold casting number for a 1976 engine but I couldn't find that exact number so I'm not sure.
Anyway, from the few numbers posted, it's looking most like a 1976 400.
Here are some diagrams which might be useful in helping you locate various numbers.
http://groups.msn.com/TransAm1978/engineidentifiers.msnwhttp://groups.msn.com/TransAm1978/cylinderheadid.msnwThe diagrams are for 1978 TA's so the numbers themselves won't match but the
locations of the numbers would be the same on a 76 engine and heads. Post any numbers that you might find and we'll help decode everything for you.
Hope this helps.
Oh, and Welcome to 78TA.com.
Added: After some further research, I discovered that the X7 engine code was used on a 1973 455 engine also but not the SD. So it is possible that you have a 455. All could be answered if you were able to get at that CID casting (driver's side of block towards the front). A 455 with either 6X-4 or 6X-8 heads is not a bad combo.
