There's no basis for that statement, as far as I know. All of the early FM and TK transmissions had a significant history of failure. Also, the newest TK is nearly 30 years old. Even the FM can be no newer than what - 1992? The Mazda, while far from perfect, was designed for use behind the 4.0 and generally held up well in that environment. It was also continuously produced from the late 80's until the end of Ranger production.
The basis for that statement is back in the day on here most of the old trans gurus agreed the TK was stouter then an FM145/146.
But no where near as strong as the M5OD.
Ive never had a 145/146 so i cant really say how shitty they are, however, i know ive beat the TKs hard and never lost one.
I had an M5OD in a 93 F150 with a 300 at one point for a few months, but they seem to hold up till they start leaking.
All im saying is i wouldnt be afraid to run the TK and then when/if it explodes swap in an M5OD. If you get the motor/trans as a unit, then by all means have at it. A TK is worthless to just about anybody as a standalone part, so why just scrap it without it being destroyed first? It might outlast the truck, even with a 4.0. Who the hell knows?
Personally you could get a frickin A4LD to stand up to a 460 as long as it was mounted in a festiva and you never went past 1/4 throttle.
Thats why i said
If you take it easy
EDIT...
Wasnt there an adaptor out there where you could run a "real" trans from a 300/302/351W behind a 2.8/2.9/4.0?
If there was id find a ZF from behind a 351 or a T18 from any of the above and never worry again
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