my $ .02:
Swapping to the 250 engine would have a number of
issues to work with. The 250 cylinder head is a reasonable and simple upgrade
with a little help. Carb and Ignition upgrades, fresh 'valve job' with a
target compression ratio for your driving and improved valve seals will wake up the 200's latent performance.
What about the carb and distributor?
'66 200 likely is the SCV/carb type distributor , 250 is probably 'later' points distributor used on late 60's early 70's Ford sixes before Duraspark was introduced. Better distrib', not 'SCV" carb dependent advance, 250 dist' has Vac and Centrif' advance (
AFAIK).- also can swap Electronic Duraspark
IF it fits...
... not sure if the '66 200 will accept later distributors with different dimension and shaft size. The casting numbers by starter will reveal actual 200 series and someone here would know for sure ...
Carb may be larger of 1100 series 'Autolite/Motorcraft'Ford, or Carter 'YF" or "RBS" .250 carb fits later 250 head's larger intake port/flange directly. '1100 is early design with higher CFM for 250 - 'RBS" was used for lower hood clearances of Maverick/Comet with 250, good chlice if Hood clearance is an issue.
'1100' carbs in front and 'RBS" carb in rear:
Carter ' YF's
Hav efun