The #1 thing you do not want to do is downsize the passage/hose/pipe, that is most definitely going to add back pressure. For my stuff I measure 1-3/8 pump to top of radiator and 1-1/2 on suction side to bottom.
On my Swamp buggy the engine sits a good distance from the radiator and I make the hoses with exactly what you say, exhaust pipe and hose clamps. Weld on a ridge for good grip and improved blow off prevention plus match up hoses from the rack at Advance auto. If you want it to look good purchase SST tubing with short lengths of rubber hose and clamps like your picture.
As for bending thin wall tubing, you need to fill it with sand to keep it from kinking and run it over a decent size radius (if you don't have a bender).
McMaster Carr is a great place to purchase short lengths of thin wall SST tubing. They stock in Atlanta and ground shipping is normally 1-2 days to Fla.
McMaster-Carr
One thing I notice on your setup is the looped hose between the two heater connections. You need to replace that with blanking plugs on each port. What you have plumbed in now is essentially a bypass of the relief valve that I covered above. So that hose is an open hole for hot engine coolant to flow back into pump suction, bypassing your radiator. Effectively defeating all of the goals here.
The thing to consider is your idle condition when engine speed and therefore pump flow is low. You want the engine to cool down at idle, but a big bypass hose makes that difficult. At high RPM that heater bypass loop is probably not an issue. The pump will flow plenty and it will normally open the relief valve to dump excess, the loop you have just keeps the relief from opening as much.
Keep that heater loop and you will come back in a year and say the engine heats up at idle but cools right down if you jump on a plane. The standard explanation will be because you added airflow to the radiator. Perhaps, but reycling hot coolant at idle sure does not help.
maybe I should just buy the holley
