Howdy Bill:
Yes, this would be a good year for a 250 builder. Depending on when it was cast the engine will likely have D5xx casting marks. The D5DE-BA head will be a flat-top type, any will have a 1.75" carb hole, 1.75" intakes and 1.38" exhaust valves. It will likely have induction hardened seats on the exhaust side, but possibly have hardened seats on both intake and exhaust. FoMoCo was in the process of changing to hard seats during this time, so you'll have to look to be sure what is there. That's why we recommend a D7 and later casting. This head will have the larger combustion chamber at 62 ccs. You mentioned installing larger valves. A larger exhaust is easily done, but to install larger intakes, know that valve shrouding minimizes the advantage of a larger intake. The small bore diameter is also part of the problem.
There is some smog stuff, again, depending on where it was destined for, but not as much as later. All can be removed and dealt with. There are bungs and taps cast into the head and the exhaust manifold that can disrupt flow, but not significantly.
Hope this helps.
Adios, David
