Define high output? The heater core has to fit into a groove in the housing, so it has to be a particular size. Rock Auto shows three heater cores, one of which is aluminum. That's about the only option I see. If you're looking for "high output" in terms of blower capability, you're going to need to upgrade your blower. Are you not getting enough heat or enough air movement?
1995 Blue YJ i-4 AX-5 33" Cepek Crushers on 2.5" Rancho lift - SOLD October 2010
1996 White ZJ i-6 auto bone stock
1987 Maroon YJ i-6 Auto (1.5" body lift. OME 2 1/2" lift. Rancho RS5000 shocks. Rancho steering stabilizer. A-Z Fabrication rock slides. More upgrades coming soon - LOL)
The "sideways" build thread: http://www.jeepforum.com/forum/f22/r...hread-2164793/