With a straight grip and held properly, your draw length will most likely be shorter on the Hill with proper form.
My draw was 29.5 on the high wrist recurve and hybrids.
When I switched to the straight grip Hill, it went to 28.5
Now it is hovering right about 28 and a little bit, as I moved up in bow weight from the 50 lb Hills to my current setup of 70-75 lbs.
It's just natural for you to lose that inch to an inch and a half. Don't fight it, plan on it and all is well.
Howard's rule (not set in stone) was whatever your last number in your draw length was the last number in your bow length, ......so for me 28 and a bit calls for a 68 inch bow. Now mind you, that measurement on Hill bows is nock to nock length so all my 68 inch HH bows are around 69.5-70 inches overall. My 70 inch HH bow is around 72 inches long. But the measurement that you go by is nock to nock length. I shoot both my 70 inch and now my 68's are where I'm sticking fr length just like Howard suggested.
To long a bow limb length for the draw length is not getting the limbs to work enough. That's why he suggested that formula. I really like the action of my 68,s a bit better than my 70 incher.
And no, the string didn't pinch going from longer to shorter nock to nock. If I were you I would try shooting both the 70 and the 68 to see what you like better. But if your not accustomed to shooting the Hill longbow, it will take you a while to develop the form. Find someone who shoots a straight grip well and learn from him.
Good luck!