If the mounting plate has slots, you should be able to line up four bolts. Like geezer says. I would be leary to rotate or hang the engine with only three bolts. There is a lot of leverage on the bolts and eliminating 25% of the strength?

I wouldn't. Phred makes a mounting plate that attaches to the side of the block. This is the best mount I have ever seen. This exposes both end of the engine and with the balance centered you can rotate the block with one hand.

Gary

That bracket will support two arms easily in one slot or drill an additional hole, but I don't think that will be nessesary. Does your adapter plate look just like this? If so study it a bit more, it should work. You can have the one in this pic free if you want to pay the postage, but its kinda heavy.