You might find that Excel Dyno V2.2 is useful when investigating performance
with a different engine and larger tires.  A can't vouch for the absolute
accuracy of the calculations but it was pretty accurate in predicting my 0 to
60 mph time with my Subaru Vanagon Westy conversion.  It also predicted that
the 0 to 60 mph time with 215/65r16 tires would be a trace quicker than with
the smaller 205/70r14 tires.  Actual measurement proved this to be true.
Further investigation indicated the reason was I would stay in more rapidly
accelerating lower gears longer to higher speeds with the larger tires and
not suffer the acceleration penalty of shifting into third as would be done
with the smaller tires.  Accelerating to higher speeds would, of course,
eventually be overall slower with the larger tires.

Excel Dyno v2.2 can be downloaded from the Yahoo Excel Dyno Users Group.