Jamis 29'er Question

I have a nice Jamis Coda Comp bike. I purchased it for a very good price and thought about taking the parts off of the frame and building a true mountain bike 29'er. Well, I've enjoyed riding the bike so much and cannot spring for anything new right now.

Here is the issue:

I can get a 700 X 45 Panaracer on the front. I can only get a 700 X 35 on the back. I've been able to find plenty of cross tires that fit it but haven't tried them yet. Have any of you had any luck using narrower tires on double-track that is somewhat rocky but no big jumps? I am a big guy and I weigh 240 pounds and am 6'3" tall.

I've recently moved to a really nice area to mountain bike on and am wondering if the tires can handle it. The wheelset is a Ritchey Mountain Wheelset with heavy spokes.

Comments?