WOW
1) Does your truck have two catalytic converters..? You made sure that both are not plugged..? My truck had a cat inside the exhaust manf..!
2) Did you get sat results on any compression test..?
3) The distributor has 3 timing adjustments for advancement A) Initial B) Centrifugal C) Vacuum Test them with idle rpm set use a laser tachometer...
4) Mikuni carburetor A) Test needle seat for leakage B) Set float adjustment C) Test the vacuum diaphragm that opens the secondary throttle plate
5) Load test the battery & coil...
I'm thinking not enough air or fuel is reaching the combustion chambers at higher speed & load and the exhaust must not be plugged (Muffler..?)
I had to replace the cracked cylinder head on my truck to solve combustion gases blowing water out the tailpipe & loss of power...
Keep at it and eventually you will fix the problem; the habit of testing before buying parts is worth the study & effort...
P.S. Follow torque specs and check tightness of all fasteners under the hood including electrical connections grounds etc. exhaust manifold nuts intake manifold nuts (my truck had 3 missing nuts on the old head) carburetor bolts, etc. etc. Install & test using all new filters in the intake casing one for air & one for blow by... Disconnect the vacuum going to the air intake duct warm up valve on the casing snorkel end remove the hose and cap it off at the vacuum source under the Mikuni carb... It will default to stay open without vacuum supply and should allow more cold air into the intake especially if it has a leak which causes it to default to closed which causes hot air to always be supplied to the intake (it happened to my truck)...Good Luck...
You deserve
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