I know the frames on 84-85 trucks are different than the IFS trucks from 86 on. if you have the whole donor truck just swap everything entirely. line up donor truck next to your rig, pull out 3.slow crap and put in all the new 3.4, look on pirate for 3.0 to 3.4 swaps there everywhere and I'm not sure but i think the motor mounts are very similar. just like 22r mounts are to 3rz. i believe going this route you could swap it over entirely then your new truck will have a factory 3.4 in an older body. you can even pull off all the engine bay brackets, mounts, etc from donor truck and it will be a clean swap
keep the rear axle for width maybe chebys in the rear, use the front axle off the 85 under the 89, go hi steer since you have IFS box already, RUF, swap IFS hubs onto axle for width up front. ch