'69 307: Large or small journal?

[70heavy]

I am also considering this with mine. 1968 + cranks are large journal. Your crank is cast nodular iron, and that is the only type for the 307. I would just find a good 350 block, as they are everywhere, although a 327 block of course, will work. Try to stay with a block earlier than 1986 unless you want to buy an adapter kit for a one to two piece rear main seal... This later block will have 4 tapped holes and a machined flat above the rear seal area.

The blocks are "basically" the same other then that with a common standard bore size of 4.001".

Watch for:

Dipstick on the side you want it.

Starter motor bolt patterns (2 different size flywheels were used)

Block with a casted number 10 over 20 by the camshaft bore is preferred as they have a high tin/nickel content which helps the block breathe and maintain a seal longer.

Good Luck