Greetings .
I was wondering if this would be a direct replacement combo for my E-maxx # 39036-4 ?
Hobbywing Ezrun 5.5T/3650KV + eZRun-60A-SL combo for 1/10 & 1/12 Car $108.65 Free Shipping @GoodLuckBuy.com
My concerns are
1 The motor bolting to the stock plate
2 Will I still be able to use the 2 battery system
3 Are the motor shafts the same dia , ?
4 Will my (2 at a time) 5400 2s Venom batteries be OK to use .

Thanks for any help folks .

I am sorry, but the e-maxx is too heavy for a 1/10 scale motor and esc. I would aim for a 2200kv motor and a 120A esc at least. I would also run with 4s lipos. There are several 1/8 scale systems out there from eg. hobbywing and other chinese brands.

Traxxas may call the EMaxx a 1/10 but it's not the same 1/10 that the motor/esc manufacturer is thinking of when it says 1/10. A truck the weight of the EMaxx needs a 1/8 brushless system which consists of a motor that's at least 36mm diameter (40mm preferred) and at least 70mm long (74mm or longer preferred) with a kv that's 2400kv or less (2000-2200kv preferable). As far as esc it needs to be at least 120amp capable, 150amp preferred and 4s capable.

The system you linked is a 3650 motor (36mm diameter, 50mm long) which means less power and the motor working harder, heating up faster. The esc is only 60amp capable which will be exceeded in a truck this weight. With only 2s you have less voltage which means less power, higher amp draw, more heat etc.

There are some people using smaller motors and esc in an EMaxx with less than 4s. Those are usually Short Course Truck (SCT) systems and on 3s. SCT systems usually have 550 motors that aren't much smaller than 1/8 motors and the esc are usually at least 80amp if not 120amp. Even so with these SCT systems in an EMaxx you have to watch temps carefully and gear for slower speeds to keep them in check. If your truck has bigger and heavier tires, aluminum parts or if you run on grass then you need gear even slower. Get a 1/8 system you'll be better off.

Can you recommend any of the combos that are out there ?
I appreciate your wisdom and actually I'd be running 4S ( 2 5400's at 2s ea. )

Something like these:

Hobbywing MAX-8 150A 6S Sensorless ESC 4274 Brushless 2200KV Motor Combo Redcat | eBay

https://hobbyking.com/en_us/turnigy-...00kv-120a.html

Hobbywing QUICKRUN WP-8BL150 Brushless ESC w/4074 2000KV 4Pole Brushless Motor Combo w/Cardfor RC Cars $167.54 Free Shipping @GoodLuckBuy.com

There are many options for 1/8 scale motors and esc.

Do I need to get a serial harness or a parallel harness to run these 2 batteries at one time ?

You want the batteries hooked up in series for 4s. Some esc's don't come with any battery connectors installed, some have a single battery connector, others have 2 battery connectors already setup in series. Pick the esc you want then solder on the battery connectors so you don't have to use any adapters.

Edit: As far as recommendations for which esc and motor, I try to steer people away from Turnigy Trackstar. The Hobbywing Max8 Funniekid linked to is about $50 more but they are proven to be more reliable, the esc is 6s 150amp capable instead of 4s 120amp, the esc is smoother, according to the pics it already has 2 battery connectors setup in series, it's just a better system.

OK , I'll go with that one .
Is it waterproof ?
If it doesn't come with a programmer , which one do I need ?? Or are they universal ?

The Hobbywing Max8 is waterproof. I'm not sure you'll need a program card as the power switch has a "set" button used for programming at least some functions. Go to Hobbywing.com, look up the Max8 and it's instructions. The instructions will tell you what program card is compatible and what functions you can program without one and what functions you need a program card to change.