1973 Other Chevrolet Models engine questions

Engine Mechanical problem
1973 Other Chevrolet Models V8 Two Wheel Drive Automatic

Hello, I just recently rebuilt my first motor which is a 1973 chevy 454. The motor is on a mighty mount engine test stand. I couldn't put the headers facing down because of the way the test stand is so I bought some block huggers and faced them upwards. I'm running into a few problems. The motor runs hot after some time, and I'm fairly sure the water pump and the thermostat are working because the bottom radiator hose is cold and the upper one is hot (so I'm assuming the thermostat is allowing the hot water to recirculate back to the radiator and the water pump is moving the water. My fear is that the head gasket might be blown. Because the coolant keeps going down, but I'm not sure if maybe it's just because the block still needs more water since it was totally empty and I should just keep filling the radiator up as it's going down. Also the stand shakes pretty good and I'm unsure if the coolant is being ejected out of the open radiator cap because of that or it's because of an internal problem. Also I'm not sure when people say the oil will be milky if it's a blown head gasket? How milky is milky? I used BR Engine break in oil for the cam breakin which is darker already and has a higher zinc amount. So my oil does look a little weird compared to the usual. On the dipstick it looks pretty good. My other serious problem is that before I did have mufflers (purple hornies) on the headers but they were almost glowing red hot. I took them off and fired the motor just to see and I get a 5 to 6 inch flame coming out of the headers almost like a drag car. Not all the time, but half the time those things light up. My timing is adjusted now where the motor seems to be idling pretty healthily. What could be the reason for this? I understand it's not good to run a motor without back pressure (i.E. The mufflers), but when I flipped the block huggers around they don't fit exactly on the exhaust ports. A little off center, will this not create back pressure? I can't find 3 bolt flanged pipe turn down mufflers to fit the huggers. Any help will be greatly appreciated. Thank you.