I used a piece of clothes hanger wire with a little hook bent into the end to figure out how deep the threaded holes were, then measured the bolts and put in ones that were about that length.

Keep in mind that any that go into the water jacket must have thread sealer on them or they will leak. That means any holes that don't have a bottom on them.

RR, 1 1/4 D is very safe, it might be interesting to you to know that bolts will develop their whole strength with two full threads engaged in the nut.

R.