BAWITHLOT

Yours are XC and the OP are XD

I reckon it did not have any lube on it when it was run in. Very unlikely to be pushrods, springs or preload when all the rest are ok. I had a rebuilt engine do that and took 13K before it failed and out of warranty by a year!

Not long enough runners, I would still be smoother with the tunnel ram!

A tunnel ram would work better if the front and rear runners don't join on the top part of the manifold! https://www.ebay.com.au/itm/FORD-CLEVELAND-302-351-2V-TUNNEL-RAM-INTAKE-MANIFOLD-12-15912-200/132798569618?hash=item1eeb695c92:g:frwAAOSw3thbVrlv:rk:29:pf:0

What sort of engine do you want to run it on!

It is your second gear syncro that has caused your box to fail. The first indication should have been crunching when going into reverse. If you have spare boxes you just take the first gear synchro out and fit it into your second gear position. I have a couple gear boxes and should be in Cowra in a few weeks if that helps!

Sometimes senders fail when they heat up!

I fitted my own conversion and had it inspected and approved by my local Gas Fitter. However I was in the Aircraft industry and work with oxygen and electrical systems! I basically copied the donor car but replaced the lines with flexible type. There were a few hassles but I complied to the fitters requirements and it passed. In the end the install was neater than any of the other professional ones I have paid for before because I could afford the time to design a tidy install instead.