Ok....well there is no junkyard around here that sells ANY engine for one price, so I find that hard to believe. I just do not see the 4200 as a viable swap in anything yet, as the proper pieces to make them work in most older vehicle applications are rare, and expensive. The easiest, and cheapest way for the majority of garage mechanics will be the old 250 and 292's, hands down, all day long, until the parts for proper swaps are available.
Look here since you don't believe me..
pricing All the parts needed to do a 4200 swap except engine mounts are now available so I don't agree with you but this since your firm in your belief I'll let this rest and let others decide on there own.
