You have to look at both motors:
v8 intake sits between the hears with heat coming from under it and had SHORT INTAKE RUNNERS.
On a L6, yes the exh & intakes are bolted together to heat the intake runners.
But if you were tosays not run the stock exh manifold, why block the heat riser and stay with a poor flowing exh manifold what do you gain?
The LONG INTAKE RUNNERS that are now running cooler the fuel will drop out of the air / fuel mix and pool in the intake floor and cause run issues.
The heat is used to keep the air / fuel atomized to the end holes, the one that run lean to start with, and if the fuel was to drop out may not get any fuel just air.

Because you are not removing the heat altogether you may be ok. Even more so if you don't drive when it gets cooler out.
Why not wire the heat riser so it will not let the hot gasses to the intake for a test. This valve when cold should let hot gases to the intake to keep the fuel atomized when the motor is cold then close to keep gasses from getting to the intake.

BTW what air filter you using? Stock with the air preheat hose to the exh manifold? Could have carb icing when cool and damp with out the heated air.
If open then you are sucking in hot air all the time so why block the intake heater?
See its all or nothing and you also need to know what each part does and what may happen if changed or removed.

They say its your truck do as you wish so if you want to block it do so.
If you find on cool days it runs like crap think about adding heat to the intake to keep the air / fuel atomized.
Dave ----

 

Dave,
Thank you for the input. You give me more to think about before blocking it off.
I am in the middle of putting this back together after cleaning this old truck up. (Grease, dirt and mud dobbers) it sat on a farm for quite a while.
I think I will take your advice and try the wiring the riser shut to start with. Here in Alabama
it does not get that cold. right or wrong I worry more about heat than cold. (maybe wrong thinking)
Once again I appreciate your input.
Alex

 

I did see where you are from so if blocked off you may not have any issues at all.
But what about that person that lives somewhere it gets cold, I lived in CT for 55 years and it got cold, that reads this and said "lets block off the heat to the intake" and then has issues but does not tell us they blocked off the heat to the intake. We then chase the issue to the carb, dist, and who know what and still not get it fixed.

I have blocked the intake on anon-Ford v8 but it was a drag race only motor and till it got a little heat into it it ran like crap.
Dave ----