I've wrestled with what you are. (no known amount, modified system, how much to you use?) and asked here and the answer is you have to experiment to find what works best, then hopefully be able to write down that amount for future use.
My experience is those honking big condensers, that were designed for R-12 with wide fins and large tubes (to minimize air flow restriction to the radiator) don't handle 134a as well as the modern parallel flow multi path condensers that are physically smaller but have a higher capacity (like the MEI 6314, it has almost 32000 BTU Cap).
I replaced the one on my IHC 9670 that filled the whole grill opening yet didn't cool well on 134a with the 6314 and I have COLD A/C.
When you are un sure of how much to 134a to use, it is always better to start out low, and work up then to put too much in.
If you have a sight glass in the receiver, you can use that as a guide. Always allow some time for the system to "settle" between adding. Be prepared to go back and tweek it esp if on a really hot day. As long as you have liquid in the sight glass, you should be close, if there is foam or lots of bubbles you are low, too high will look the same as "just right" so work up to it from too low, not down from too high.
With my engine fan running, the temps in the mid to upper 90's in a hot parking lot, the inlet to the condenser is HOT but the outlet is just warm, slightly above ambient That shows that amost all the heat is removed from the 134a.
Be glad you have an old system with a reciever and Tx valve. They are a lot more forgiving of amount than are orifice tube systems. No condenser will work well without good air flow, but if you are cooling ok at idle, than I don't think airflow is a problem.
Make your adjustments with both bunk and cab units on.
Sight glass can be misleading if the condenser is not doing its job. Compare pressure vs. temp and see if it is in the ball-park. There is a pressure/temp chart for both 134a and R-12 on this fourm.
If the air across the condenser is 90 deg, you want to see a high side pressure of no more than 40 deg higher (130 deg ) or around 225 psi for 134a. The lower you can get the high side temp/pressure the better cooling. In otherwords, the more heat the condenser can remove, the lower the high side condensing temp/pressure and the better it will cool. So if you can get the outlet of the condenser to be 105-110 deg F (instead of 130F), the pressure will be 135-147.The same holds for low side. compare you vent temp with the low side pressure