If you were going to make your own adaptor plate, it would be best to first know what the radius is from the crankshaft of the Y-block to the engine-bellhousing mounting bolts as a starting point. Knowing that the radius is 8 inches for the I-block, you can see that you will have two radii layed out on a sheet (the smaller radius being the I-block and the larger being the Y-block. Then you can lay out your bolt hole pattern.
How would the starter work though? You have a manual trans with a smaller flywheel than that used for the Y-block. Will it even be possible to mount the starter so that the starter Bendix touches the flywheel gearteeth ? need to closely look at cast iron ribbing and bumpouts at the back of the Y-block to make sure that a mounting bolt for the manual trans bellhousing doesn't hit the ribbing before driving the bolt home. All kinds of things to think about !
