Wait
Boss or case mount is a function of the engine and engine only not the alternator.
Wait to buy your engine. The older engines (more than 20-25 years old) will not have the boss (flat pad with threaded holes) and requires you to mount the alternator off the bolts that holds the case halves together. The Boss mount is the better mount by far.
Electronics do not take much electricity. The high current drain items are landing lights, nav lights, strobe lights, some electronic ignitions, fuel pump, flap motor, comm radio during transmit and transponder. I assume you are going to have night capability (lights)? It is a simple matter of adding all the items up to determine your critical load requirement (night approach).
When you talk 35 amp I assume you are talking about Van's 35 amp alternator with external regulator. I suggest you avoid it. From what you are telling me you will be up well over 20 amps pushing +30 amps or more. A 35-amp alternator will be hard pushed. Also that 35-amp alternator is of an old design with an external fan that Van does not supply. The newer small case ND alternators (40-55 amps) have dual internal fans and are much more reliable. (Van's 35 amp is a ND alternator but of older design with an external fan that Van leaves off, which is not a great idea from a heat stand point. These alternators last about 100-300 hours max ). Van only sells the 60 amp newer ND alternator and does not sell the newer small ND alternator, which is what I suggest.
A 40-55 amp ND alternators are in a small frame package. When you go to 60 amps you go to a larger heavier alternator. You can't size your alternator or determine your alternator size until you buy your engine and finalize all the items in you planes electrical system. You may need a 60 amp alternator, I don't know. The idea of building is keeping it light, so if you can save a few pounds with a lighter alternator than do that. If you need the higher amp capacity than you need the larger alternator. You will have to size it.
Good luck, but it sounds like you are a ways out from buying an alternator. I also have a good piece of advice. Buy a new alternator. You can buy them off of eBay sometimes or from large auto electric supply houses (not auto zone). Rebuilt ones are OK and do have a warranty, but they are not like a brand new ND alternator. If you shop around it is no more than a rebuild. New 60 amp ND alternators are harder to buy new, so if you go 40-55 amp ND you should buy new.
Internal voltage regulated alternators like the New ND have internal OV protection. The theory is a rare failure (in theory only) that is unlikely to happen will cause a runaway voltage condition. There are no documented cases to prove this and only urban legend and stories with no facts. Some will tell you with religious fervor that you SHOULD not, MUST not, and cannot use an internally regulated alternator with out something called a crow bar (OV protection added on top the alternators internal OV protection). The chance of an OV problem, with a new ND alternator, is slim to none and you don?t need a crow bar, which can cause more problems than solve. If some one says you needed it, make them prove that there is a problem, other than a friend of a friend said he had an OV problem with a ND alternator. When you can track these claims down it turns out to be a Cessna with an old fashion external voltage regulated alternator. ND alternators have had very few problems. The problems they have had are small and not the massive over voltage condition melt downs that is falsely rumored that can or have occurred. Put a (CB) Circuit breaker on the output lead of the alternator (called the B-lead). Put this B-lead (CB) in your panel; if you have an indication of Hi or Low volts you can pull the CB to protect the system manually. You should always have a Hi/Lo volt idiot light or warning of some kind.
Keep it simple, keep it light, stick to the plans. G
PS
HINT: you should try to have your alternator load never continuously run at more than 50% of it's rated capacity (e.g., cruise night condition) and not more than approx 75% of it's max load (night approach). The reason is heat. The less heat the better the reliability. You put any alternator in and run it at 100% of its rated capacity for long periods it will fail earlier. My max continuous load is about 27 amps night cruise, day cruise 9.3 amps. My peak intermittent load is 32 amps for night t/o and approach. I have a 45-amp small frame ND alternator with internal VR and fans and no other OV devices.
