I would run premium gas in it, but the rule of thumb is, use whatever
the lowest is that doesn't cause it to ping under load (accelerating up
an on-ramp, etc.)  It depends on how it's tuned, as well.

Do you know if the engine was rebuilt with hardened valve seats?  If so,
you won't need lead substitute.  If not, which would be odd these days,
you do.

I don't mean to besmirch the regulars here, but there is a better group
in which to ask questions about this car -
alt.hi-po.big-block-ford-mercury.  Post this question there.  You'll get
advice from a group of seasoned pros.

My '71 351C 4V M-code runs great on 91 octane which is roughly
equivalent to 99 octane of yesteryear.  The timing is set about
12 BTDC and the car never fails to start on the first try even below
freezing.  The engine is very responsive.  This is California so the gas
may even contain alcohol.  In Yosemite last year I paid $3.84/gallon
of high test.  But with a car like yours, you don't worry about the price
of gas.

mike

With regard to the unleaded fuel issue, the chief concern has been for the
possibility of exhaust valve recession (sink or seat wear) when engines
designed to run on leaded gasoline are operated on unleaded gasoline. If
your engine was completely overhauled recently it may have had hardened
valve seats installed therefore making this is a moot issue.

Typically, most passenger are subjected to service too mild for this issue
to cause problems. The problems typically only occur in older engines when
operating under *sustained* high speeds and loads.

With regard to the octane issue, the easiest answer to your question
regarding the proper octane fuel to use is to use the octane rating as close
as possible to what the manufacturer suggests for the engine. This mindset
assumes you have not made
major modifications to the engine (compression ratio/timing). Since your
engine was completely overhauled recently it may also have had a reduction
of the compression ratio from the original engine design therefore making it
more "friendly' to current premium fuels (91-93 octane).

I am not sure what version of the 351C engine you have since the 351C was
not offered from the factory in the 1969 Mustang model year.

FYI:
2-barrel H-code 351C featured a 9.5:1 compression ratio and a recommended a
minimum fuel octane level of 94
4-barrel M-code 351C featured a 11.0:1 compression ratio and a minimum
recommended fuel octane level of 99.8

Yet another $.02 worth from a proud owner of a 1970 Mach 1 351C M-code
(searches for the highest octane I can find) @
http://community.webshots.com/album/18644819fHAehGJAjt