Mustang Tech

i have an fmx in a 70 mustang hard top behind a 357 long rod Cleveland, comp cam 230/230, balanced ,ported and bigger valves, small chamber heads nothing ott ,9 inch 3:25 gears lsd,holley sniper and hyperspark . after doing these upgrades i put my fmx back in after taking out my 5spd out . the fmx had frictions about 9 years
ago did about 1000ks then i pulled it out to go 5spd , when i put it in recently i had a shift kit put in only a trans go shift improver, but now it seems to work fine but if you get under it and kick back top or accelerate from start when it changes gear 2nd to-3rd it free revs and slips till it either grabs or you back off ,under normal driving its fine im thinking all these improvements have stressed it out too much and cant handle the new power and torque, just looking for some other opinions

How much torque are you talking about? Nothing in the combo seems out of the ordinary and I would expect a stock FMX to handle that.
Have you double checked your fluid level? Do you know which frictions were used? Might be worth putting a gauge on your line pressure port or the test port for 3rd gear if it has one to make sure you're maintaining pressure.

oils good, its the stock box it came with and has been sitting for about 9 yrs look it works fine just slips under heavy load , but it was behind a tired old 351

I guess it's possible that the frictions have gone bad from sitting for a long time with no fluid. My dad has a mild 351C XD and his FMX never had issues holding it.