I'd say it depends more upon your skills as a fabricator, materials availability, and local inspection regs than anything else. Here in CA, you have to have everything bloody approved by the state Air Nazis (since 1966!) - but in most of the rest of the country, it's a non-issue.
A cold air intake may be made from some large pipe or ducting, fit it into the air intake hose, and fit an open element cone filter onto the other end. If you really want to do it right, get some sheet aluminum and fabricate a "fence" around it to keep engine heat at bay - insulate it if you like.
Throttle body spacers are of limited utility on RENIX anyhow - since the intake manifold is not the best design going. You'd probably be better off fabricating an intake manifold that allows for better air distribution - particularly to #1 and #6 cylinders. Have a look at OBD-II 4.0 manifolds for clues.
Switch to Ford-style "four-jet" fuel injectors (
www.five0motorsport.com, I think) and you'll see better fuel atomisation, and better power as a result.
A bored throttle body is likewise of limited utility - since the 4.0 doesn't run fast enough to need that much air. However, a little cleanup work can go a long way - get a Cratex abrasive kit, and clean up the inside of the throttle body bore (there is probably some casting flash in there, and some sand-cast surfaces) and the intake runners in the cylinder head, if you feel like removing/disassembling the cylinder head to do it. DO NOT actually remove enough material to change the shape of things - just clean it up. You'd be surprised what can come of a little detailed cleanup work (and head porting should not be done without a flowbench anyhow, for complex work.)
Do bear in mind that these engines are not designed and built for high-end horsepower, they're designed and built for low-end "grunt." If you want horsepower, you may want to switch to a Camaro or a Corvette. If you want torque for hauling and passing, you're on the right track.
What gears do you have now, and what do you plan on going to? If you don't know your gear ratio offhand, the AW4 gets 3.55 gears behind it, and the BA-10 gets 3.07's. Some "trailer tow" packages get 4.10 gears. You should also be able to find a longish tag under the cover bolts on one axle or the other, and it will have the gear ratio "in the clear" on it - lower line, right side, I believe.
5-90
