[onetrikpony]

I kind of like the old build-it-yourself books. I'm not seeing any huge conflict in using the old Vehicles and Robot books to build gear for my game.

PB - looks like it can just be ignored.

HP - Based on surface area in Vehicles and Robots, which seems to make better sense for mechanical things than using HP = ST ~ Mass

Power Plants - Robots gives definitive Joules ratings for each E-Cell! This is exciting and handy unless it turns out to violate the precepts in 4th edition.
Ultratech is pretty vague on power production and storage so I can't see any conflict.

Weapon Stats - in the 3e books I think they can be largely ignored there are plenty of choices and examples in UT.

LARGE vehicles - I havn't really looked at this. But I can't imagine any utility in building something large than a Main Battle tank. the old system might break down for something like large submarines or areal craft. I'd probably just go to Space Ships for the build and Vehicles for the accessories.

I don't want to break the game. Are there any pitfalls I'm missing?

Thanks for the advice. :)