the oversize heads will help... my nt-16 in tall grass would easily peak over 300 on a warm day... this won't necessarily hurt the engine as long as your not leaning it out... make sure for good smoke... I run an oversized head on one of my trucks with a .21 nitro star in tall grass and it is fairly cool... mid 260's WOT, but it is more of a hassle to tune... the cen oversized purple head looks good and runs well but tuning is a bit more difficult to set... I now run the os .18 cv and temps in med to low grass only 240-250 with wheels pointed to the sky...
as far as a cost effective way to lower chassis, try using small piece of fuel tubing inside the shock to limit the length.. i used about 1/3 inch and lowered the height to where the dog bones and cvd's were parallel to the ground... (control arms were straight out) keep in mind this limits jumping unless you change to a stiffer spring and or change shock piston rate and fluid... I experimented and found that a #1 shock valve and 30 weight oil with a med spring works for the best balance overall... just don't sky the truck off huge ramps...
Per Cen Racings advice, I stopped using the diff cup mod and spent more time adjusting the suspension geometry and found that I don't break anymore diff cups and also, the dog bones and cvd's are lasting longer.. I was breaking cvd and dog bones with the diff mod... The diff cups are designed to break before the more expensive dog bones and cvd's even though they should last longer... hopefully they will eventually come up with a stronger cup design...