I have an early 250 HEI distributor with the external coil. I'm wondering will the later cap with integrated coil on cap fit my distributor?

Yes, it will fit. You may have to dose minor mods to the wires.

I don't think so, the coil in cap does not provide a connection for a coil plug wie to go to the external mounted coil.

If it is the big cap they will interchange. The coil connects directly to the cap. No coil wire is used. There were some early HEIs that used a smaller cap more the size of the points style cap. I can't post pictures here anymore Or I'd show some that I have.

here's my '78 HEI refurbished. First replacement high quality cap, rotor, coil cover, and wire loom:



Coil in place:



Bottom of stock coil:



Everything in place and wire layout. Didn't have the adjustable vacuum advance on yet (adjustable up to 20 degrees, so I put 10 initial on it) and you can't see my springs which I think are currently two of the Accel red, or maybe one red and one blue:



Also I pulled the whole distributor out, disassembled and cleaned it. Then lubed it up real good so it is nice and snappy on the advance. After several decades it was really gummed up and the mechanical advance would stick. Big improvement.

If someone else comes across later maybe it'll help when running an HEI. Thought I'd share a budget friendly tech tip.

Since the recommended plug wire size is 8mm and most aftermarket sets are 7 or smaller, unless you're buying more of a premium set. Figured guys might like to know that stock Ford Fairmount wires are 8mm, I get them for an '80 when replacing wires on the Jeep and they fit well with the straight 6 and are usually less than $25 for the set.