[quote]Will a C6 bolt onto the 63 block ?<hr></blockquote>
Yes it will, but if the car was mine, I would have the FMX overhauled by a COMPETENT AND KNOWLEDGEABLE SHOP.
The conversion will give you trouble in linkage and driveshaft length at the least. Also be advised that about the year of your BIRD, FORD changed the engine mount pads on the block so take this into consideration if you want to use a later engine.
The 390 can be warmed over easily and still appear to be factory assembly.