The 260 in many peoples minds is a 289 with a smaller bore. Ford tech manuals
state that the maximum overbore for a 260 is .030. Many people have used this
limit with success, but it is the variation in the manufacturing that allows
some block to accept a .040 overbore. Overheating is a common problem when
the 260 is bored beyond .030 as will as the risk of cracks developing in the
cylinder wall. The 4 inch bore and any possible overbores are possible on the
289 because Ford change the position of each cylinder providing adequate
space without risk of overheating or having durability issues. A good
reference, Mustang & Ford Small Block V8, 1962-1968, Vol. 1 & 2, and worth
the $49.95 is available from Amazon.com and other book stores.
All is not lost if you want to retain the original block. It is expensive,
but the block can be sleeved. Another option is to locate a 5 bolt pattern.
While not a direct bolt in, the Tigers United site has the info you will need
to make the modifications for the 289.
Dr. Moonstone
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