Jay, if you're concerned about the balance, use aluminum bolts, they hardly weigh anything.
I have my own balancing story. I put a 455 into my 67 Olds 442 that was basically a slightly freshened up junkyard motor out of a car with an automatic. I put a steel manual tans flywheel on it.
The first time I ran the motor while I was actually sitting in the drivers seat, I noticed a distinct vibration, that changed with engine speed. I wanted to make sure that it was not from an engine miss, so I did a power balance test by pulling one spark plug wire at a time and checking for constant RPM drop. That came out fine, so then I Checked the exhaust temp on each header tube with a handheld infrared thermometer. It seemed that all 8 where hitting fine. So I pulled the fan belt to rule out fan, etc. No change noted.
Then I dug out the old flex-plate that had come on this motor, and sure enuff, it had a big ol' counter weight welded to it. I put the flexplate and a damper on the crank out of the old motor ( which was sitting on the work bench) to check the orientation of the flex plate weight in relation to the damper weight, and they were about 15 degrees apart.
THen I looked at the flywheel in the car, and balancing holes where drilled in about the same location as the counterweight on the flexplate. Weird. The flywheel is drilled with a few different clutch mounting patterns, and those holes are threaded all the way thru the flywheel to the front. So I used one of the unused holes to bolt 5 ounces of lead the front of the flywheel, in the location where the counterweight on flexplate was. After checking for clearance, I started the motor and it shook like a belly dancer. So then I took the weight off and moved it 180 degrees and started the car. It was way smoother, but still vibrated a small bit. I experimented with weight untill at 3.25 ounces, the motor was smooth as a babys behind. THen I made up a 3.25 oz. steel weight that I carefully ground to fit the contour of the flywheel to lock it in, and bolted it on with a locktighted grade 8 bolt. I think it is safely there to stay, and it works perfectly. I can't beleive I balanced my own motor without removing it from the car!