help with motor selection

[dearl]

I have been doing some book work on my 72 Grady project, trying to plan ahead for the motor install, but I'm stuck on the single or dual outboard delima. My plan is to try and get at least 250 h.p on this transom, its rated for 245 on the tag. I know once I install the 1 1/2 coosa bluewater in her she will hold alittle more. I have given a lot of thought about the weight of the bracket also, most i have seen range from 115-140 lbs. depending on the application. I will stay away from a heavy 4 stroke if i have to, but i don't want to sacrifice eff. and speed tooo much! Right now the boat weighs about 2400 lbs with the inline 6 and OMC outdrive, it is a walk around cuddy cabin, and is built really, really well to accept a bracket. Any Help?