I had the same problem when trying to put my motor onto my stock perches. My truck was I6 that got switched to sbc (with the I6 perches on the rear holes). When I put my LS conversion plates in place and set the motor in, I was 1/2" short on room.
I decided I also wanted to move my engine forward; so I bolted the mounts an plates to the motor, then bolted the perches directly to the mounts.
With the whole thing up on the cherry picker, I set the engine into plate and used the OLD front hold as my new rear hole. I used new hardware to hold it into place, and then my mounts were welded into place.
