Try minnflyer's suggestion on moving the whole engine forward. He's always got good suggestions. I've done the same with a plywood spacer sandwiched between the firewall and the engine mount. 1/8 plywood sounds like it might meet your needs. Just drill a new set of holes and stick it in there.
For mounting the engine to the mount, I'm using socket bolts with washers and nylon insert locknuts. 8/32 and 6/32 for most engines seems good. Much easier to install these bolts with a ball driver than using the stock sheet metal screws that come with the engine mounts.
I've bought several collections of these fasteners now at
www.rtlfasteners.com.
Waagbuck