No instuctions? It would be nice if you had the thrust line, but we can cheat a little if we have to. Mount the landing gear. Point the fuse straight up and hook the landing onto something that will keep the firewall straight and level. Now put the cowl on. First thing we are going to determine is the distance from the firewall to the opening of the cowl. Write that measurement down. Mount your spinner and prop to the engine. Next, set the engine in the engine mount. Measure from the back of the mount (the part that sits on the firewall) to the backplate of the spinner. Hopefully you'll have enough room on the mount to slide the engine where it needs to be. I usually put the backplate about 3/16" away from the cowl. Do you have enough room on the mount to have clearance for the spinner backplate? If no, you'll have to space the engine mount off of the firewall. You can use plywood for this. If yes, were ready to mount it. When you have enough length for spinner clearance, you can mount the engine to the mount. Now were ready to move on.
Leave the cowl on, the prop, and spinner on the engine. Set it into the cowl and on to the firewall. Does the firewall have DOWN and RIGHT thrust built in? If so great. Just center the engine in the cowl. The spinner and prop will help you do this. Look at the fuse from the both sides with the prop at the 12 and 6 o'clock position. Look centered? Next move the prop to the 3 and 9 o'clock postition. Look at the fuse from the top side. Look centered?
If it's all centered out, use a long drill bit to drill the holes into the firewall though the holes in the mount. If you don't have a bit long enough, mark it some how and remove everything to drill. I'll try and find some pics to help you out.