The reason its so difficult is because the front axle of an Eagle is mounted to the engine. In a Spirit, you can buy fancy adaptors that have a 258 bolt pattern on one side and a V8 bolt pattern on the other. In a Jeep you get different metal L brackets and bolt them to the frame where the 258 brackets used to be. Both are bolt and go solutions. In an Eagle, however, its not at all the same. You have to make very fancy metal brackets that incorporate the axle supports on both sides of the engine. Terry has done a great job at making a very complex pattern that incorporates everything you need.
The next tricky part is that Eagles have the engine offset two inches to the passenger side to make room for the front driveshaft. A 258 doesn't have anything on that side of the engine, so it doesn't matter. On an AMC V8, however, there is a exhaust manifold on that side. You have to cut the old 258 mounts off the crossmember and weld on new mounts closer to the driver's side. The engine will still be offset, but it will be offset less than before. The lateral spacing of the new mounts is critical, so you have to weld it after everything is bolted together and squeezed in. I'll be supporting the engine with an engine hoist with the crossmember removed. It will be bolted to my factory T5 with its manifolds, front axle, transfer case, driveshaft, and full motor mounts all attached to the engine. I will then put the crossmember under the engine, shimmy it until I can tell where to weld, and then mark the crossmember and take everything back apart. I will be using a much stronger transmission than the T5 but I need it installed during this step to position the engine where it will be compatible with any factory transmission and driveshaft.
I've put information about the transmission options elsewhere in this forum. For an Automatic you definately want to use the TF727 because it shares the same linkages as our factory TF998s. Only use a TF998 or T5 if you have a 304. The 360 and 401 need stronger transmissions and rear axles to go with them. Most people will get the TF727 and AMC 360 together from a full size Jeep. Beware of the TH400, its the other common transmission in full size Jeeps. It must be avoided since its not compatible with kickdown levers, floor shifters, transfer cases, or even axles.
Plan on getting a Dana 44 rear axle from a Cherokee. The Commanche Dana 44 will also work well, but they have spring perches on the wrong side and no shock mounts. A Grand Cherokee Dana 44 has rear calipers, but they too need all their old mounts cut off. Ideally you should weld on new spring perches and Eagle shock mounts on any axle, but the Cherokee ones kinda work as is. One of the shocks will be on the wrong side of the axle and the perches are abit off, but it will work. I've also heard of some people doing well with Ford Explorer axles. The Jeep community also has directions to easily convert the Cherokee drum brakes to the Grand Cherokee calipers.
Flywheels are hard to come by. They're the same thickness as a 258 flywheel and have the same bolt pattern, but they are balanced specifically to the engine. I will use an Advance Adaptor between an NV3550 and a T5 bellhousing to retain the original flywheel and clutch linkage. To install my 6 speed NSG370 I will need to bring a 4.0 flywheel to a machine shop and have it rebalanced to match the flywheel or flexplate my engine came with. A 4.0 flywheel is a quarter inch thinner and I must use the factory slave cylinder because the NSG370 has an integral bellhousing.
I have a set of Spirit AMX headers that I will attempt to use, but I honestly don't think they will work. Everyone I know of has used the factory 360 manifolds (360 manifolds are bigger and should be used even if you have a 304) because only one pipe then needs to snake its way around the control arm supports and driveshaft. Its considerably more cramped in an Eagle than a Spirit.
The oil filter will not fit. You need a remote oil filter kit. The only one I know of is sold by bull-tear. There is also a rumor that a Jeep oil pan will not work since the original mockup was done using a V8 from a passenger car. A Javelin guy told me that the pans are actually the same. I'll have to flip my Jeep V8 over and try bolting the axle on when I get home to find out for sure. The high capacity Jeep pans are fatter and they definately won't fit.
I have pictures of a 304 installed differently in an Eagle. That one was fabricated to hang the axle from the front of the transmission. Those mounts are crude, hideous, and obviously weaker than Terry's mounts. For the amount of time and aggravation it takes to mock these up its well worth buying his. The only advantage I see of making your own mounts is that you can position the axle further from the engine to accomodate a larger oil pan or a lift. Terry put the axle as close to the engine as possible. Even then I think it would be better to buy his and modify them than to start from scratch.