The bellhousing on a 289 is for a 157 tooth flywheel and the clevelands oil pan rail kind of likes the 164 tooth bell housing(D1TE) and flywheel for starter clearance. No big deal parts wise.
One big thing is the output shaft on the HEH-P is a 25 spline and not the normal small block 28 spline. It may be a little weak.
I was wondering about the 25 spline output. I have a 25 spline yoke and it has worked fine behind a built 289 for about 7 years. Now the question. Is this a 6 cylinder toploader or one from an earlier year or is there some other reason altogether?