For a trialer I'd start with an F20 chassis & axles if I were you. There short, there narow (but not so narow as to be unstable), there stronge & they don't need lifting to be efective (alowing your centre of gravity to remain low). If you build your own body onto it you can lower the centre of gravity some more. There are two weak points: 1) the free wheeling hubs shear their bolts off of the main hub, if you allow the bolts to work loose. 2) the front spring hanger of the rear springs is on a tubular out-rigger, and twists off after a few years abuse. Welding a stay bar back to the main chassis prevents this. Also lopp the rear chassis off right after the rear most spring hangers & put the rear cross member (or a peice of box section) back in between the chassis rails. (Leave about 3 inches of chassis, other wise the swinging shackels are the very back of the car. This meanes that if you back into a tree, the rear sprig takes the impact, causing it to bend). Now there's two routes to go for a mighty machein.

1) There is the mighty meaty way. A rover 3.5 (or 3.9 efi) engin can be fitted. You'll need either a rang rover bell housing & an F50/75 bellhousing, or a range rover compleat box (the strongest & best for trialing is the 3 speed auto).
To go the Daihatsu box method. Cut the F50/75 (Ally) bell housing, and the range rover bell housing to the right length the fit between the v8 & the box, then weld it back together. It dosn't have to be perfict for a trialer. It dosn't go that fast. You will also need to put the Daihatsu clutch releas padles into the rangi clutch plate. If your using an F75 box instead of an F50/20 box you'll also need to make up prop shafts, as the F75 uses wider UJ flanges.
To go the rangi box method (the better idiea, though probably more expensive). Place the engine & box into the car where it fits. Make up props to bridge the gap between the box & the axles. The best way to do this is cut the tube part of the existing props. Then weld them back up so you have two props with Daihatsu cups at one end & rangi at the other. Make sure you get the lengthes right.

2) This is the fun car. And very capable. (You'd be suprised). Get any front wheel drive (transverse engined) engine & gearbox, compleat with drive shafts. You need an engine which will sit nearer the back of the car than the front, when placed with the gearbox centred between the axles of the rolling chassis. An auto box is easier as you wont have to make up linkages for the gear stick. You'll have to weld up the diff in the gearbox, otherwise you'll get stuck by spining only one wheel, like a defender/rangi with the center diff unlocked. Now make up pros by welding Daihatsu props onto the existing drive shafts from the transvers engine. Place your drivers seat in front of the engine. And to finish off either fidle breakes on the back, or a welded rear diff. This vehicle does not need an over abondence of power(a 1300 metro engine is quite suficiant, though ther's no reason not to use a 2ltr injection if you like) as it is quite light. Also with the whaight destributed more towards the back it makes light work of climbing up over obsticals.

Of course if you just want to have a laugh, with very little time spent building it. Just put a roll cage on any petrol Daihat's and go for it. I'd still say find yourself an F20, as there small size means you'll get through places that many others (exept Suzuki's of course) can only marvel at. I personally have an F20, which is mainly standard, that has been used for trialing for over 10 years now, with very little mantenance.

Any other questions, please ring me. I'll mail you my number on this sites internal mail.